The Anthropocene is functionally and stratigraphically distinct from the Holocene.

Human activity is leaving a pervasive and persistent signature on Earth. Vigorous debate continues about whether this warrants recognition as a new geologic time unit known as the Anthropocene. We review anthropogenic markers of functional changes in the Earth system through the stratigraphic record. The appearance of manufactured materials in sediments, including aluminum, plastics, and concrete, coincides with global spikes in fallout radionuclides and particulates from fossil fuel combustion. Carbon, nitrogen, and phosphorus cycles have been substantially modified over the past century. Rates of sea-level rise and the extent of human perturbation of the climate system exceed Late Holocene changes. Biotic changes include species invasions worldwide and accelerating rates of extinction. These combined signals render the Anthropocene stratigraphically distinct from the Holocene and earlier epochs.

Insulin increases endothelin-1-evoked intracellular free calcium responses by increased ET(A) receptor expression in rat aortic smooth muscle cells.

While insulin is known to promote vascular smooth muscle (VSM) relaxation, it also enhances endothelin-1 (ET-1) secretion and action in conditions such as NIDDM and hypertension. We examined the effect of insulin pretreatment on intracellular free calcium ([Ca2+]i) responses to ET-1 in cultured aortic smooth muscle cells (ASMCs) isolated from Sprague-Dawley (SD) rats and measured ET(A) receptor characteristics and ET-1-evoked tension responses in aorta obtained from insulin-resistant, hyperinsulinemic Zucker-obese (ZO) and control Zucker-lean (ZL) rats. Pretreatment of rat ASMCs with insulin (10 nmol/l for 24 h) failed to affect basal [Ca2+]i levels but led to a significant increase in peak [Ca2+]i response (1.7-fold; P < 0.01) to ET-1. The responses to IRL-1620 (an ET(B) selective agonist), ANG II, and vasopressin remained unaffected. ET-1-evoked peak [Ca2+]i responses were significantly attenuated by the inclusion of the ET(A) antagonist, BQ123, in both groups. The ET(B) antagonist, BQ788, abolished [Ca2+]i responses to IRL-1620 but failed to affect the exaggerated [Ca2+]i responses to ET-1. Saturation binding studies revealed a twofold increase (P < 0.01) in maximal number of binding sites labeled by 125I-labeled ET-1 in insulin-pretreated cells and no significant differences in sites labeled by 125I-labeled IRL-1620 between control and treatment groups. Northern blot analysis revealed an increase in ET(A) mRNA levels after insulin pretreatment for 20 h, an effect that was blocked by genistein, actinomycin D, and cycloheximide. Maximal tension development to ET-1 was significantly greater (P < 0.01), and microsomal binding studies using [3H]BQ-123 revealed a twofold higher number of ET(A) specific binding sites (P < 0.01) in aorta from ZO rats compared with that of ZL rats. These data suggest that insulin exaggerates ET-1-evoked peak [Ca2+]i responses via increased vascular ET(A) receptor expression, which may contribute to enhanced vasoconstriction observed in hyperinsulinemic states.

Altered paracrine effect of endothelin in blood vessels of the hyperinsulinemic, insulin resistant obese Zucker rat.

OBJECTIVE: Earlier, we reported that high insulin incubation in vitro leads to increased ETA receptor expression in cultured rat aortic smooth muscle cells (Diabetes 1998, 47: 934-944). Our later observation of enhanced endothelin-1 evoked vasoconstriction in aorta from the hyperinsulinemic obese Zucker rat indicated that this interaction might also be relevant in vivo. To further examine the relationship between insulinemia and endothelin, we characterized endothelin receptor expression and endothelin-1 peptide levels in vascular tissues and plasma from young and old obese Zucker rats. METHODS: 12 and 40-week-old Zucker obese and lean rats were used. Plasma endothelin-1 levels and endothelin-1 peptide content in the mesenteric artery and in the thoracic aorta were examined by radioimmunoassay. Messenger RNA levels of endothelin-1 peptide and ETA and ETB receptors were examined in the aortic and mesenteric vessels using RT-PCR. RESULTS: Obese rats from both age groups had significantly higher plasma levels of insulin (4-10 fold), total cholesterol (2-3 fold), triglycerides (10-fold), and glucose (approximately 1.5 fold) than their lean counterparts. There was a trend toward worsening lipoproteinemia and glycemia, but improved insulinemia with age in the obese rats. In association with these changes, obese rats exhibited attenuated endothelin-1 peptide and preproET-1 mRNA levels, but conversely elevated ETA and ETB receptor mRNA levels in both aortic and mesenteric vessels. CONCLUSION: These data suggest that vascular tissue from the metabolically dysregulated obese Zucker rat exhibits attenuated endothelin-1 peptide production and elevated endothelin receptor levels. Since elevated insulin levels have been linked to increased endothelin receptor expression, it is plausible that hyperinsulinemia upregulates endothelin receptors contributing to elevated vasoconstrictor responses to endothelin-1 in this model of obesity and hypertension.

Hepatic vasopressin receptor: differential effects of divalent cations, guanine nucleotides, and N-ethylmaleimide on agonist and antagonist interactions with the V1 subtype receptor.

Previously, we reported that magnesium (Mg2+) enhanced the binding affinity of arginine vasopressin [( 3H]AVP) to a single class of sites in rat liver microsomes. In the present study we have examined the effects of divalent cations and guanine nucleotides on the binding characteristics of both the nonselective agonist and the V1 receptor-selective antagonist, d(CH2)5Tyr(Me)-[3H]AVP, to microsomal and plasma membrane fractions of rat liver. At a subsaturating concentration (100 pM) of [3H]AVP, divalent cations increased specific binding in a concentration-dependent manner with the following rank order of potency: Co2+ greater than Mn2+ greater than Ni2+ greater than Mg2+ greater than Ca2+ = control. The maximal effect for Mg2+ was evident at 1 mM, a physiologically relevant concentration. In contrast, binding of the V1 receptor antagonist (at a subsaturating concentration of 10 pM) was inhibited by divalent cations, the rank order of potency being Mn2+ greater than Co2+ greater than Ca2+ greater than Mg2+ greater than Ni2+. The inhibitory effects of divalent cations were of lesser magnitude (up to 60%) compared to the stimulation of agonist binding (up to 700%). Mg2+ enhanced the affinity of [3H]AVP (Kd was decreased from approximately 2 nM to 133 pM), while the affinity of the [3H]V1 antagonist was decreased (Kd was increased from 10 to 95 pM). Scatchard analysis of saturation data (Mg2+ present) revealed similar maximum binding values for the binding of radiolabeled agonist and antagonist, indicating that AVP receptors in rat liver are mostly of the V1 subtype. Competition experiments between V1/V2-specific AVP analogs with either the radiolabeled agonist or antagonist also indicated the presence of predominantly V1 receptor sites in rat liver microsomes. The properties of plasma membrane receptor sites were similar to those of the microsomal sites, except that the density of receptors was higher in the former. In both equilibrium and competitive inhibition experiments GTPase-resistant analogs of guanine nucleotides, GTP gamma S and GDP beta S, decreased the affinity of the agonist for the receptor, but not that of the antagonist. Treatment of membranes with 0.2 mM N-ethylmaleimide (NEM) reduced the maximum binding of [3H]AVP and abolished the GTP gamma S-evoked decrease in agonist-binding affinity. In contrast, antagonist binding was unaffected by NEM. NEM pretreatment failed to influence the divalent cation-dependent increase in agonist-binding affinity. The results provide direct evidence for the existence of a high and a low affinity state of the hepatic V1 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a specific, high affinity [3H]arginine8 vasopressin-binding site on liver microsomes from different strains of rat and the role of magnesium.

A single class of high affinity, low capacity, specific binding sites for [3H]arginine8 vasopressin (AVP) has been characterized in a plasma membrane-enriched microsomal fraction of the rat liver. Specific binding was saturable, linear with protein concentration, reversible, and 40-65% of the total binding. Binding at 25 C achieved a plateau after 30 min of incubation, whereas at 4 C, equilibrium was reached more slowly, and the level of binding was reduced. The presence of magnesium (Mg2+) in the assay medium enhanced the affinity of specific binding, while calcium and higher levels of sodium and potassium decreased binding. Scatchard analysis of binding in the presence of Mg2+ (5 mM) revealed an apparent mean +/- SE equilibrium dissociation constant (Kd) of 0.29 +/- 0.08 nM, with a maximal site density (Bmax) of 150.4 +/- 25.0 fmol/mg; in contrast, the Kd was 1.93 +/- 0.33 nM and the Bmax was 113.4 +/- 40.0 fmol/mg in the absence of Mg2+. No significant differences in Kd and Bmax were observed among membrane fractions derived from spontaneously hypertensive rats, Wistar-Kyoto rats, Long-Evans rats, and Brattleboro rats. Plasma levels of AVP were similar in spontaneously hypertensive, Wistar-Kyoto, and Long-Evans rats, but AVP was not detectable in the plasma from DI rats. Competitive inhibition of specific [3H]AVP binding by unlabeled AVP and related peptides showed the following Ki values: AVP, 0.19 nM; LVP, 1.7 nM; oxytocin, 41.4 nM; desamino AVP, 0.38 nM; [1-(beta-mercapto-beta, beta-cyclopentamethylene propionic acid) 4-Val,8-D-Arg] VP2-(O-methyl)tyrosine]AVP, 1.8 nM; desglycinamide AVP, 2.2 microM. The neuropeptide metabolite of AVP [[pGlu4,Cyt6] AVP-(4-9)], angiotensin II, and other unrelated peptides did not displace [3H]AVP, demonstrating the specificity of AVP and its related biologically active peptides for this binding site. Moreover, the rank order of potency for displacement of [3H]AVP binding by these various peptides parallels their reported glycogenolytic activity in liver and/or their agonistic or antagonistic potency in vascular smooth muscle. Finally, the Mg2+-induced increase in the affinity of [3H]AVP for this liver binding site is similar to the reported effect of Mg2+ on the contractile responses of vascular smooth muscle to AVP (i.e. increased affinity). The results are consistent with the interpretation that the high affinity receptor site characterized in rat liver microsomes is of the V1 type.

Antihypertensive effect of vasopressin withdrawal in young and adult spontaneously hypertensive rats.

The extent to which age influences the effect of prolonged intravenous infusion and withdrawal of arginine vasopressin (AVP) on blood pressure was investigated in spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto controls (WKY) of 6-, 10-, 14-, 18-, and 22-week age groups. The pressor response to AVP (20 ng/kg/min for 3 hours) was relatively well maintained in WKY but showed an age-dependent tachyphylaxis in SHR. After cessation of the infusion, arterial pressure of SHR fell in all age groups. In contrast, withdrawal of AVP in WKY resulted in little or no hypotensive response. Thus, a withdrawal-induced antihypertensive phenomenon (WAP) to AVP was specific to SHR. The magnitude of the WAP was significantly correlated with the level of initial blood pressure in SHR (r = -0.81, p less than 0.001). The magnitude of the tachyphylaxis during the AVP infusion was also correlated with the level of initial blood pressure in SHR (r = -0.66, p less than 0.001). Accordingly, a significant correlation was found between the magnitude of the WAP and the degree of tachyphylaxis to the pressor activity of AVP in SHR (r = 0.69, p less than 0.001). The significance of this is unknown, but it might mean that a common underlying mechanism existed in the expression of the tachyphylactic phenomenon and the WAP in SHR. Finally, an apparent enhancement in the baroreceptor reflex sensitivity was observed in both SHR and WKY during the infusion of AVP, but the magnitude of this enhancement appeared to be greater in SHR than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity in vascular smooth muscle responsiveness to angiotensin II. Role of endothelin.

We compared the role of endothelium and of endothelin in mediating the vasoconstrictor responses to angiotensin II (Ang II) in three vascular smooth muscle preparations--aorta, mesenteric artery, and tail artery--isolated from adult male Sprague-Dawley rats. The vasoconstrictor potency for Ang II in blood vessels with endothelium varied in the following rank order: aorta > mesenteric artery > tail artery. Although the maximal tension responses to Ang II were similar for mesenteric and tail arteries, it was significantly lower in aorta. Endothelium removal led to a leftward shift in the concentration-response curves to Ang II in the aorta but a rightward shift in the mesenteric artery. Strikingly, Ang II failed to evoke tension responses in tail artery in the absence of endothelium. The endothelin-A (ETA)-selective antagonist BQ-123 blocked the responses to Ang II in a noncompetitive manner, with partial and complete attenuation of responses in the endothelium-intact mesenteric and tail artery preparations, respectively. In contrast, BQ-123 did not affect the responses to Ang II in the aorta. BQ-123 also failed to affect the responses to Ang II in endothelium-denuded mesenteric artery rings. The Ang II type 1 (AT1) receptor-selective antagonist losartan competitively blocked the responses to Ang II in the three tissues (pA2, 8.3 to 8.7) when endothelium was present. These data suggest that there are endothelium-dependent regional variations in vascular tissue sensitivity to Ang II.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of an endothelin antagonist on hemodynamic responses to angiotensin II.

We determined changes in blood pressure, cardiac output, and total peripheral conductance evoked by intravenous infusions of angiotensin II (Ang II) in conscious, unrestrained normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) before and after pretreatment with bosentan, a nonselective endothelin antagonist. Blood pressure was recorded by radiotelemetry and cardiac output by ultrasonic transit-time flow probes. Bosentan per se failed to affect basal blood pressure and evoked only small changes in cardiac output and total peripheral conductance in both strains. The pressor effects of Ang II were exaggerated in SHR compared with WKY. Strikingly, bosentan pretreatment blunted the increases in blood pressure, the fall in cardiac output, and the decreases in conductance evoked by lower doses of Ang II but not higher doses of the peptide. This effect was observed in both rat strains but was more pronounced in SHR. These data suggest that endothelin contributes to the hemodynamic effects of Ang II in both SHR and WKY and that endothelin may contribute to the exaggerated pressor responsiveness of SHR to Ang II.

Pretreatment with vasodilator or V1-antagonist abolishes vasopressin withdrawal hypotension in spontaneously hypertensive rats.

In spontaneously hypertensive rats (SHR), the cessation of a 3-h intravenous infusion of arginine vasopressin (AVP, 8 mU/kg per min) resulted in a large and prolonged fall in arterial pressure (46 +/- 7.5 mmHg below basal levels). Pretreatment of SHR with the specific V1-receptor antagonist, [1-(beta-mercapto-beta, beta-cyclopentamethylene propionic acid), 2-(O-methyl)-tyrosine] AVP (d(CH2)5Tyr (Me)AVP, 8 micrograms/kg followed by 0.05 micrograms/kg per min) abolished the pressor response to AVP, and markedly reduced the subsequent hypotensive response following the cessation of the AVP infusion. The hypotensive response to AVP withdrawal was abolished also when phenylephrine hydrochloride (PE, 20 nmol/kg per min) elicited a blood pressure rise during the course of AVP infusion in rats with V1-receptor blockade. Finally, the concurrent administration of sodium nitroprusside (30 micrograms/kg per min) not only counteracted the pressure rise during AVP infusion, but also prevented the hypotensive response that normally accompanied the withdrawal of AVP. These findings demonstrate that neither V1-receptor activation nor blood pressure elevation alone was sufficient to generate a hypotensive response to the withdrawal of AVP; rather, both V1-receptor activation and a blood pressure elevation associated with the activation of these receptors were essential to the hypotensive response that followed the withdrawal of AVP in SHR.

Role of endothelin and vasopressin in DOCA-salt hypertension.

1. The relative roles of endothelin (ET) and vasopressin (AVP) in the regulation of blood pressure (BP), cardiac output (CO) and total peripheral resistance (TPR) were investigated in the early stages (24 - 31 days) of development of hypertension in the conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rat model. 2. BP was recorded with radiotelemetry devices and CO with ultrasonic transit-time probes. TPR was calculated from the BP and CO recordings. The contributions of endogenous ET and AVP were studied by infusing [d(CH(2))(5)(1),O-Me_Tyr(2),Arg(8)]-vasopressin, a V(1)-receptor antagonist, and bosentan, a mixed ET(A)/ET(B) receptor antagonist (Study 1). Vascular responsiveness was estimated from the changes in TPR evoked by i.v. infusions of ET-1 and AVP (Study 2). 3. In study 1, infusion of bosentan reduced TPR and BP dramatically in DOCA-salt hypertensive rats but not in SHAM control rats, and this effect was greater when the AVP system had been blocked. In contrast, the V(1) receptor antagonist alone failed to change TPR and BP in DOCA-salt hypertensive rats. However, subsequent infusion of the V(1) receptor antagonist during the plateau phase of the response in bosentan pretreated DOCA-salt hypertensive rats led to significant decreases in both BP and TPR. 4. In study 2, TPR and BP responses to ET-1, but not AVP, were greater in DOCA-salt rats than in control rats. CO responses to ET-1 or AVP were similar in the two groups. 5. The results suggest that both ET and AVP play a role in the maintenance of TPR and BP; when one system is blocked the other compensates. However, the magnitude of the contribution to the hypertensive state appears greater for ET than for AVP. Enhanced vascular responses to ET appear to contribute to this greater role.

Differential effects of phosphoramidon on contractile responses to angiotensin II in rat blood vessels.

1. Cumulative concentration-tension response (C-R) curves to angiotensin II (AII), big endothelin-1 (big ET-1), ET-1 and arginine vasopressin (AVP) were determined in endothelium intact-ring preparations of aorta, mesenteric artery and tail artery isolated from adult male Sprague-Dawley rats in the presence or absence of the neutral metalloprotease inhibitor, phosphoramidon. 2. The order of sensitivity of the three rat vascular smooth muscle preparations to AII, big ET-1 and ET-1 was aorta > mesenteric artery > tail artery whereas that for AVP was reversed, namely, tail artery > mesenteric artery > aorta. 3. Phosphoramidon blocked the responses to AII in a concentration-dependent manner, whereas even very high concentrations of phosphoramidon (100 microM) failed to affect the tension responses evoked by ET-1 and AVP in all three preparations. Low concentrations of phosphoramidon (10 microM) produced significant increases in EC50 values for AII in tail artery (P < 0.01) and mesenteric artery (P < 0.05) but not in aorta. The rank order of sensitivity to the inhibition by phosphoramidon was tail artery > mesenteric artery > aorta. Phosphoramidon-evoked rightward shifts in the C-R curves to AII were much higher than those to big ET-1 in both mesenteric artery and tail artery. 4. In endothelium-denuded preparations, AII failed to evoke any increases in tension in tail artery while the responsiveness of the mesenteric artery to AII was reduced significantly relative to endothelium-intact tissues with a rightward shift in the C-R curve and a decrease in the maximal response. On the other hand, the C-R curve to AII was shifted to the left in aorta following removal of the endothelium.Importantly, ET-1 and AVP evoked vasoconstrictor responses were unaffected by the inclusion of a high concentration of phosphoramidon (100 microM) in endothelium-denuded aorta and mesenteric artery.5. The results suggest that AII-evoked tension responses of blood vessels such as tail artery are completely endothelium-dependent; in relatively larger blood vessels such as mesenteric artery they are partially endothelium-dependent while in much bigger conduit type blood vessel such as aorta, they are endothelium-independent. It is concluded that the vasoconstrictor responses to AII in mesenteric artery and tail artery may be mediated by the release of endothelins from the endothelium by increased formation from big ET, an effect that is blocked by phosphoramidon.

Calcium antagonizes the magnesium-induced high affinity state of the hepatic vasopressin receptor for the agonist interaction.

1. The present study describes the role of Ca2+ in the regulation of the hepatic vasopressin V1 receptor. With low concentrations of Ca2+, there was a small increase in [3H]-arginine vasopressin [( 3H]-AVP) binding, but above 10 mM, Ca2+ decreased the binding of this agonist. In contrast, low concentrations of Mg2+ were associated with a dramatic concentration-dependent increase in [3H]-AVP binding, reaching a maximal effect of 650% above control at concentrations ranging between 1-5 mM. At higher concentrations of Mg2+, the stimulatory effect of this cation was less pronounced, falling to 210% of control at 100 mM Mg2+. Strikingly, Ca2(+)-inhibited the stimulatory effect of Mg2+ in a concentration-dependent fashion. 2. Saturation binding data revealed that Ca2+ (2 to 10 mM) per se promotes the high affinity conformation of the V1 receptor for the agonist binding with the KD decreased from a control value of 2.3 nM to 0.5 nM in the presence of 10 mM Ca2+. This effect was attenuated with an increase in Ca2+ above 10 mM. With an increase in Ca2+ to 20 mM, however, the Bmax for [3H]-AVP binding was decreased. Ca2+ also decreased the high affinity/high capacity state (KD 100 pM) of the receptor induced by 1 mM Mg2+ for agonist interaction. 3. [3H]-V1 antagonist binding was inhibited by both Ca2+ and Mg2+. The IC50 values (mean +/- s.e. mean) for Ca2+ and Mg2+ were 32 +/- 8 and 53 +/- 9 mM respectively. Maximal inhibition achieved at 100 mM was 29% for Ca2+ and 42% for Mg2+. Both cations decreased the affinity and increased the capacity of the V1 receptor for the antagonist. 4. The results suggest that the divalent metal ion binding site(s) modulated by Mg2 + is also accessible to Ca2 +. Although Ca2 + opposes the powerful stimulatory effects of Mg2 + on agonist binding, the effects of Ca2+ and Mg2 + on the B,,x of [3H]-AVP binding were different, suggesting that the divalent cations may bind to two different sites, thereby regulating the affinity and the capacity characteristics of the V1 receptor.

Angiotensin II elevates cytosolic free calcium in human lung adenocarcinoma cells via activation of AT1 receptors.

Angiotensin II (Ang II), bradykinin (BK), and endothelin-1 (ET-1) evoked alterations in cytosolic free calcium, [Ca2+]i, levels were determined using fura-2 fluorescence methodology in a human lung adenocarcinoma cell line (A549), a non-neoplastic lung cell line and a small cell lung carcinoma cell (SCLC) line. Ang II and BK evoked a rapid, concentration-dependent transient increase in [Ca2+]i in A549 cells. The peak [Ca2+]i increases attained with Ang II (1 microM) and BK (1 microM) were 3- and 4-fold higher, respectively (P < 0.01) than the basal [Ca2+]i values. This effect of Ang II was completely abolished by inclusion of losartan (DuP 753), an AT1 subtype selective antagonist. Removal of extracellular Ca2+ from the incubation medium led to significant diminution of the peak [Ca2+]i response to Ang II but not to BK. In contrast to Ang II and BK, ET-1 failed to evoke an increase in [Ca2+]i levels in A549 cells. Neither Ang II nor ET-1 evoked any appreciable increase in [Ca2+]i levels of non-neoplastic lung cell and SCLC cell lines. These data confirm that the human non-small cell lung cancer cells (A549) selectively express AT1 subtype receptors for Ang II that are functionally coupled to Ca2+ mobilization from both extra and intracellular sources.

Pneumadin-evoked intracellular free Ca2+ responses in rat aortic smooth muscle cells: effect of dexamethasone.

The direct vascular effect of pneumadin (PN) was determined by studying the changes in intracellular free calcium ([Ca2+]i) levels in cultured rat aortic smooth muscle cells maintained between the second and fifth passages. PN evoked a rapid, concentration-dependent, biphasic increase in [Ca2+]i. The [Ca2+]i level rose from a basal value of 108 nM to a maximum increase in peak value of 170 nM. Although the level of maximal [Ca2+]i response evoked by PN was less than with other vasoactive agonists, it was more potent (EC50 0.5 nM) than even endothelin-1 (EC50 3.1 nM). At concentrations > 100 nM, [Ca2+]i elevations induced by PN above basal levels were no longer observed. Pretreatment with dexamethasone (100 nM for 24 hr) resulted in a significant increase (P < 0.01) in the peak [Ca2+]i response (310 nM) to PN. However, the biphasic pattern in the peak [Ca2+]i responses encountered with increasing concentrations of PN remained unaffected. The exaggerated [Ca2+]i response to PN was abolished by preincubation of cells with either the glucocorticoid antagonist mifepristone (RU 486) or the protein synthesis inhibitor cycloheximide. Inclusion of either an AT1 antagonist (losartan), a V1 selective vasopressin antagonist (d(Ch2)5 Tyr (Me) AVP), or an alpha-adrenoceptor antagonist (phentolamine) failed to affect the increases in [Ca2+]i induced by PN. PN-evoked increases in inositol 1,4,5-trisphosphate levels paralleled the [Ca2+]i changes. These data suggest that PN increases Ca2+ mobilization in rat aortic smooth muscle cells via activation of phospholipase C coupled receptors. This effect is up-regulated by dexamethasone.

Withdrawal-induced antihypertensive effect of vasopressin in Doca-salt hypertension.

Arterial pressure and heart rate were recorded in three groups of conscious unrestrained rats before, during and after a three-hour intravenous infusion of arginine vasopressin (AVP; 20 ng/kg/min). Before infusion of AVP, basal arterial pressure was 152 +/- 3.2 mm Hg for the Doca-salt hypertensive group, 117 +/- 3.2 mm Hg for the uninephrectomized control group, and 111 +/- 2.1 mm Hg for the sham-operated control group. The changes in arterial pressure at each of the time points during the three-hour infusion of AVP were not significantly different among the three groups. Cessation of the AVP infusion was associated with a dramatic and prolonged fall of arterial pressure below preinfusion basal levels in the Doca-salt hypertensive group: pressure was 58 +/- 5.0 mm Hg below basal levels within 60 minutes of stopping the infusion and was still 45 +/- 5.6 mm Hg below four hours later. Pressure in this group was 107 +/- 5.7 mm Hg five hours after stopping the infusion. In contrast, the fall in pressure in the two control groups was small. Five hours after stopping the infusion, pressure was 108 +/- 2.3 mm Hg in the uninephrectomized group and 102 +/- 1.7 mm Hg in the sham-operated group. Thus, five hours after stopping the infusion, blood pressures among the three groups were not significantly different. The Doca-salt hypertensive group had been rendered normotensive.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of autonomic function in the antihypertensive effect of vasopressin withdrawal in spontaneous hypertension.

The importance of the autonomic nervous system (ANS) in the withdrawal-induced antihypertensive phenomenon (WAP) to arginine-vasopressin (AVP) in the spontaneously hypertensive rat (SHR) was investigated by studying the response to AVP infusion and withdrawal in the presence and absence of ganglionic blockade. In the absence of autonomic blockade, AVP infusion (20 ng/kg/min for 3 h) produced similar pressor responses initially in the SHR (45 +/- 2.0 mm Hg) and in the Wistar-Kyoto rat (WKY) (43 +/- 3.3 mm Hg). The pressor response was not well-maintained in SHR, falling to 21 +/- 0.8 mm Hg above preinfusion basal values by the end of the 3-h infusion. Cessation of the infusion was associated with a dramatic decrease in pressure below basal levels in the SHR: 3 h after the withdrawal of AVP, pressure had dropped from the preinfusion level of 171 +/- 4.0 mm Hg to 141 +/- 4.5 mm Hg, whereas pressure had returned to control levels in the WKY. In the presence of the ganglionic blocking agent, the changes in pressure both during and after the AVP infusion were similar in SHR and WKY: 3 h after the withdrawal of the AVP infusion, pressure was only slightly reduced below preinfusion levels in SHR (from 112 +/- 4.6 to 100 +/- 2.9 mm Hg) and WKY (from 76 +/- 2.7 to 75 +/- 2.6 mm Hg). The results suggest that an intact ANS is important to the expression of the WAP in the SHR, in that the WAP may require a withdrawal of sympathetic tone.

Total peripheral conductance mediates antihypertensive effect of nonpeptide mixed endothelin receptor antagonist in deoxycorticosterone acetate-salt hypertensive rats.

The relative contribution of cardiac output (CO) and total peripheral conductance (TPC) to the changes in blood pressure (BP) evoked by an intravenous injection of bosentan, a nonpeptide mixed endothelin (ET) antagonist, were investigated in conscious unrestrained deoxycorticosterone acetate (DOCA)-salt hypertensive rats and sham-control rats. Blood pressure was recorded by radiotelemetry devices and CO by ultrasonic transit-time flow probes. Bosentan significantly reduced BP (from 141 +/- 3 to 111 +/- 3 mm Hg) and increased TPC (from 1.19 +/-0.1 to 1.72 +/- 0.2 mL/min/kg/mm Hg) in DOCA-salt hypertensive rats, but not in sham rats. An increase in CO opposed the BP-lowering effect of the antagonist. The results demonstrate that the role of ET receptors in the maintenance of the hypertensive state in the DOCA-salt model of hypertension is exerted at the level of the resistance vessels and not on factors that regulate cardiac output.

Effect of losartan on angiotensin II-mediated endothelin and prostanoid excretion in humans.

Angiotensin II (Ang II) stimulates renal prostanoid and vascular endothelin-1 (ET-1) release. Most known Ang II effects are mediated by AT1 receptors. Our aim was to determine whether AT1 receptor activation mediates Ang II-evoked renal prostanoid and ET-1 release. Eleven healthy men were randomized in a crossover, double-blind fashion to receive 100 mg/day of losartan or matching placebo, for 8 days. Blood and urine were sampled before and after a 2-h infusion of Ang II at a rate previously determined to increase mean arterial pressure (MAP) by 25 to 30 mm Hg in each subject. After a 14-day washout, subjects received the alternate treatment. Pretreatment with losartan had little effect on baseline MAP, but increased plasma renin activity, and virtually eliminated the pressor response to Ang II infusion. Angiotensin II significantly increased prostanoid excretion after placebo; the prostanoid response to Ang II was even greater after losartan. Plasma ET-1 was not altered by Ang II infusion, with or without losartan. In contrast, urine ET-1 excretion rate decreased to 40% of baseline after Ang II but not after losartan pretreatment; losartan alone had no effect. We conclude that Ang II decreases renal ET-1 synthesis and release through the AT1 receptor. In contrast, Angiotensin II-mediated renal prostanoid synthesis does not require activation of AT1 receptors. These findings indicate that AT1 receptor antagonists could provide renal protection through indirect mechanisms.

Plasma endothelin levels and vascular responses at different temporal stages of streptozotocin diabetes.

While alterations in the release and action of endothelial derived vasoactive factors such as endothelin- and endothelial derived relaxing factor (EDRF) occur in diabetes mellitus, the nature and direction of these changes is controversial. We examined the role of diabetes duration on endothelin- plasma levels and vasoconstrictor responses to endothelin-1 in aortic rings from streptozotocin-induced diabetic rats. Endothelin-1 plasma levels were attenuated at 2-weeks, but conversely elevated at 14-weeks, after diabetes induction. Similarly, maximal vasoconstriction to endothelin-1 in aorta was exaggerated in 2-week, but attenuated in 14-week diabetic rats. Also, sensitivity of aorta to endothelin-1 was enhanced in the 14-week group. Neither nitric oxide synthase inhibition with nitro-L-arginine-methyl-ester, nor endothelium removal affected alterations in maximal vasoconstriction, but both abolished changes in sensitivity to endothelin-1. Endothelium dependent (acetylcholine-evoked) vasorelaxation responses were attenuated in 14-week, but not 2-week, diabetic rats, while endothelium independent (sodium nitroprusside-evoked) responses remained unchanged. Together, these data indicate a deficiency in EDRF production in the 14-week group. Elevated plasma glucose and attenuated insulin levels were present in both groups, but plasma cholesterol and triglyceride levels were elevated only in 14-week rats. We conclude that differences in the pre-existing duration of diabetes differentially affect both plasma levels and action of endothelin-1. These changes might be linked to coincident diabetes duration dependent changes in EDRF production and plasma lipid levels. Such a shift in the production and action of endothelial derived relaxing and contracting factors might contribute to the characteristic early and late stage cardiovascular complications of diabetes mellitus.

Vanadate treatment normalizes exaggerated vascular smooth muscle responses in the obese Zucker rat.

The effect of oral vanadate treatment on isometric tension responses were examined in aortic rings isolated from obese and lean Zucker rats. Rats from both strains that were either maintained on food ad libitum or pair-fed were included to serve as controls. Higher plasma insulin and glucose levels and exaggerated aortic tension responses to endothelin-1, methoxamine, and KCl observed in obese Zucker rats were normalized in vanadate treated, but not pair-fed, rats. These data suggest that abnormal vascular responses in obese Zucker rats can be normalized by vanadate treatment in a manner at least partly independent of food intake.