Human insulin-mediated enhancement of vascular beta-adrenergic responsiveness.

Insulin may play an important role in the physiological and/or pathophysiological regulation of the cardiovascular system. Defects in insulin secretion and insulin receptor responsiveness have been associated with increased peripheral resistance and hypertension. The mechanisms linking these events remain unclear. To assess the effect of insulin on beta-adrenergic-mediated vasodilation, we examined aortic ring segments obtained from normotensive male Wistar and spontaneously hypertensive rats. Vessels were maximally preconstricted with phenylephrine (3 mumol/L). Relaxation was induced by either isoproterenol (10 mumol/L) or sodium nitroprusside (10 nmol/L), and the relaxant response was followed for 20 minutes. Insulin exposure did not alter phenylephrine-mediated constriction. However, insulin mediated a dose-dependent increase in isoproterenol-induced relaxation, to a maximum of 120 +/- 4% of baseline isoproterenol-mediated relaxation, with an EC50 for insulin of 32 pmol/L in aortic rings from Wistar rats. Insulin exposure also did not alter nitroprusside-mediated relaxation. In contrast to the results obtained in rings from Wistar rats, insulin did not enhance isoproterenol-mediated responses in rings from spontaneously hypertensive rats. Thus, insulin mediates a selective enhancement of vascular beta-adrenergic responsiveness in aortas from normotensive but not hypertensive animals.

Epinephrine enhances neurogenic vasoconstriction in the rat perfused kidney.

Epinephrine has been implicated in the genesis of some forms of hypertension. We have investigated the effects of epinephrine on vasoconstrictor responses evoked by adrenergic stimuli in the isolated perfused rat kidney. Low concentrations of epinephrine (2.5 - 5 X 10(-9) M) increased the amplitude of vasoconstrictor responses evoked by electrical stimulation of the renal adrenergic nerves. These concentrations of epinephrine had no effect on the basal perfusion pressure of the kidney or on the amplitude of vasoconstrictor responses evoked by exogenous norepinephrine. The potentiating effect of epinephrine persisted after infusion of the amine had ceased. Kidneys that had been perfused with 3H-epinephrine accumulated radioactivity, which could then be released by renal nerve stimulation. Cocaine (3 X 10(-5) M) reduced the renal accumulation of 3H-epinephrine and abolished both the persistent potentiating effect of the amine and the release of radioactivity evoked by subsequent nerve stimulation. The potentiating effect of epinephrine infusion was abolished by the beta 2-selective adrenergic receptor antagonist ICI 118,551 (3 X 10(-8) M), but not by the beta 1-selective adrenergic receptor antagonist atenolol (10(-6) M). These results indicate that concentrations of epinephrine that can be achieved during acute stress can enhance the amplitude of neurogenic vasoconstrictor responses. This effect appears to be mediated via a prejunctional beta 2-adrenergic receptor. The persistent nature of this effect may be due to the neuronal accumulation and subsequent release of epinephrine.

Genetic aspects of variability in superficial vein responsiveness to norepinephrine.

Venoconstriction of the dorsal hand vein by local norepinephrine infusion was measured by the linear variable differential transformer method in 15 healthy unrelated subjects and eight pairs of monozygotic and six pairs of dizygotic twins. Incremental norepinephrine infusion produced dose-related venoconstriction. In unrelated subjects the doses of norepinephrine constricting basal vein diameter by 50% (ED50) ranged from 3.9 to 120.5 ng/min. There was a positive linear relationship between doses of norepinephrine infused and local steady-state plasma concentrations of norepinephrine achieved in each subject. The reciprocals of the slopes of these dose-concentration relationships, which reflect local norepinephrine clearance (disposition) in the vein, ranged from 0.47 to 1.86 ml/min. Plasma concentrations of norepinephrine associated with reduction of basal vein diameter by 50% (EC50) ranged from 1.4 to 110.2 ng/ml, with variability similar to that of ED50. There was a very high level of concordance in ED50, EC50, and clearance of norepinephrine within pairs of monozygotic twins but not within dizygotic twins. Differences in pharmacokinetics of infused norepinephrine exert a minor impact on overall intersubject variability. Genetic aspects of "tissue responsiveness" (i.e., vascular alpha-adrenoceptor response, smooth muscle contractility, and endothelial function) appear to be largely responsible for the wide intersubject variability in venoconstrictor responsiveness to norepinephrine.

Studies on the adrenomedullary dependence of kappa-opioid agonist-induced diuresis in conscious rats.

1. The dependence of kappa-opioid agonist-induced diuresis, upon an intact and functional adrenal medulla in conscious rats, was investigated in order to test the hypothesis that the diuresis is mediated by a blood-borne 'diuretic factor', of adrenomedullary origin, released by kappa-opioid receptor stimulation. 2. Confirming previous observations, adrenal demedullation significantly attenuated diuretic responses to the kappa-opioid agonists U50488H, ethylketocyclazocine (EKC) and tifluadom, but did not affect basal urine output, furosemide-induced diuresis or the antidiuretic response to the mu-opioid agonist, buprenorphine. Naloxone abolished U50488H-induced diuresis, confirming an involvement of opioid receptors. 3. Transfusion studies established that blood, from intact rats treated with U50488H, induced diuresis in intact and demedullated recipient rats, whether or not the recipients had been pretreated with naloxone. However, blood from demedullated rats treated with U50448H was unable to induce diuresis when administered to intact or demedullated recipients. 4. It is concluded that kappa-opioid agonist-induced diuresis is dependent upon an intact and functional adrenal medulla and appears to be mediated by a blood-borne 'diuretic factor' of adrenomedullary origin.

The effect of adrenal demedullation on cardiovascular responses to environmental stimulation in conscious rats.

Circulating plasma adrenaline has been implicated in the facilitation of neurogenic pressor responses and development of hypertension. Bilateral adrenal demedullation in rats did not affect body weight, urine output, urinary electrolyte (Na+, K+ and Cl-) excretion, nor plasma corticosterone concentration, indicating the selective nature of the demedullation procedure. Adrenal demedullation did induce significant reductions in adrenal catecholamine content, plasma adrenaline levels, resting blood pressure and heart rate in conscious rats, but did not affect alerting-induced increases in blood pressure. The adrenal medulla and circulating plasma adrenaline appear to contribute to the maintenance of resting cardiovascular parameters, but would not appear to be involved in nor facilitate the cardiovascular responses to environmental stimulation.

The effect of bilateral adrenal demedullation on vascular reactivity and blood pressure in spontaneously hypertensive rats.

Bilateral adrenal demedullation of juvenile spontaneously hypertensive rats attenuated, but did not prevent, the development of hypertension. Neither did it affect the subsequent vascular reactivity to phenylephrine though it significantly reduced the vascular effects of sympathetic nerve stimulation. Demedullation of adult spontaneously hypertensive rats did not alter blood pressure, but did attenuate the pressor responses to both alpha-adrenoceptor agonists and sympathetic nerve stimulation. In acutely demedullated adult rats, vascular reactivity to sympathetic nerve stimulation, but not to exogenous amines, could be restored by slow i.v. infusion of adrenaline in a dose-dependent manner. The results support a possible facilitatory role for adrenaline in sympathetic neurotransmitter release, both during the development of genetic hypertension and in vascular responses to sympathetic nerve stimulation.

On the mechanisms of kappa-opioid-induced diuresis.

In conscious saline loaded rats, the kappa-opioid agonists tifluadom, U50488, and ethylketocyclazocine, given subcutaneously, induced a characteristic diuresis which could be antagonized by naloxone. Bilateral adrenal demedullation significantly reduced adrenal gland catecholamine content and plasma adrenaline levels, but did not significantly affect plasma corticosterone levels, indicating that the adrenal cortex remained both intact and functional. Seven days following bilateral adrenal demedullation, the subcutaneous administration of the kappa-agonists no longer induced diuresis. However, demedullation did not affect the diuretic response to frusemide or clonidine, nor did it affect the antidiuretic response induced by the mu-opioid agonists morphine and buprenorphine. Adrenal catecholamines do not appear to be involved in kappa-opioid-induced diuresis, since pretreatment with propranolol, prazosin and idazoxan did not affect the diuretic response in intact animals. The results indicate a link between the adrenal medulla and kappa-opioid-induced diuresis and suggest that a peripheral mechanism may also be involved in mediating this effect.

Presynaptic receptors in hypertension.

The physiological status of presynaptic receptors, regulating sympathetic neurotransmitter release, remains subject to debate. Nevertheless, pharmacological techniques have shown presynaptic alpha-adrenoceptors, mediating a negative feedback inhibition of neuronal noradrenaline (NA) release, and presynaptic beta-adrenoceptors mediating a positive feedback facilitation. Decreased presynaptic alpha- or increased beta-adrenoceptor responsiveness might be expected to result in enhanced per-pulse release of NA and may contribute to hypertension development and maintenance. A potential role in hypertension development, but not its maintenance, has been established for presynaptic beta-adrenoceptors. Attempts to identify altered presynaptic adrenoceptor responsiveness in hypertension have, however, been inconclusive.

The distribution of NPY-containing nerves and the catecholamine contents of canine enteric nerve plexuses.

Our previous study demonstrated that PYY was a major neuropeptide in the canine enteric nervous system, the present study defines the locations of NPY-containing enteric neurons. NPY-positive nerve cell bodies and fibers were numerous in gastric and pyloric myenteric plexuses as were positive nerve fibers in antral and pyloric muscle layers, pyloric sphincter muscle layers, and surrounding blood vessels. In contrast to findings for PYY, there were considerably fewer NPY-positive nerve cell bodies and fibers in the canine ileum and colon. Noradrenaline was the predominant catecholamine in all plexuses, the rank order of its contents being: deep muscular > submucous > myenteric plexus. The dopamine/noradrenaline ratio was constant in all plexuses; adrenaline was present in minor amounts. PYY is the more abundant neuropeptide in ileum and colon, and NPY has a greater presence in the gastric antrum, pylorus, and surrounding blood vessels. PYY and NPY may play different functional roles in the GI tract.

Cardiovascular responses to intraventricular adrenaline in spontaneous hypertensive rats.

The effects of intracerebroventricular (i.c.v.) injections of adrenaline on the blood pressure and heart rate of spontaneous hypertensive (SH) rats have been investigated. Adrenaline induced dose-related falls in blood pressure and heart rate in both conscious and urethane anaesthetised rats. In conscious rats, the hypotension and metoprolol, but were unaffected by pretreatment with phentolamine, piperoxan, fluphenazine or methysergide. However, in urethane-anaesthetised rats, the hypotension and bradycardia induced by i.c.v. adrenaline was not significantly affected by i.c.v. pretreatment with propranolol or oxprenolol, while metoprolol significantly antagonised only the bradycardia. Piperoxan, fluphenazine and methysergide were also without effect. Pretreatment with mecamylamine (i.p.) abolished the cardiovascular depressor effects of i.c.v. adrenaline in both conscious and urethane anaesthetised SH rats. It is concluded that the cardiovascular depressor effects of i.c.v. adrenaline are mediated by central adrenoceptors in SH rats and that, in conscious rats, these depressor effects may be mediated by central beta-adrenoceptors rather than alpha-adrenoceptors.

Beta 2-adrenoceptors mediate adrenaline's facilitation of neurogenic vasoconstriction.

Acute bilateral adrenal demedullation significantly reduced pressor responses in pithed spontaneously hypertensive rats. The subsequent infusion of adrenaline (50 ng/min i.v.) significantly enhanced neurogenic pressor responses without affecting those induced by noradrenaline. The adrenaline-induced enhancement of neurogenic pressor responses was unaffected by pretreatment with the beta 1-selective adrenoceptor antagonist atenolol (0.3 mg/kg i.v.), but was completely abolished by pretreatment with the beta 2-selective antagonist ICI 118551 (0.01 mg/kg i.v.). Pressor responses to noradrenaline remained unaltered.

A comparison of the cardiovascular effects of centrally administered clonidine and adrenaline in the anaesthetized rat.

Intracerebroventricular (i.c.v.) injections of clonidine and adrenaline-induced hypotension and bradycardia in urethane anaesthetized spontaneous hypertensive rats. The hypotension induced by clonidine (3 microgram i.c.v.) was antagonized by pretreatment with the alpha-antagonists piperoxan, which also antagonized clonidine-induced bradycardia, and yohimbine. The hypotension and bradycardia induced by adrenaline (10 microgram i.c.v.) were unaffected by alpha-antagonist pretreatment, while beta-antagonist pretreatment with (--)-propranolol or metoprolol was effective against adrenaline but not clonidine-induced hypotension and bradycardia. Pretreatments with the histamine H2-receptor antagonists metiamide and cimetidine antagonized clonidine but not adrenaline-induced hypotension. These data indicate that different central mechanisms are involved in mediating the hypotension and bradycardia induced by centrally administered clonidine and adrenaline and do not, therefore, support the hypothesis that the hypotensive effects of clonidine (i.c.v.) are mediated by central adrenaline receptor activation in urethane-anaesthetized spontaneous hypertensive rats.

Adrenaline and the development of genetic hypertension.

The effects, on the development of a raised blood pressure in spontaneously hypertensive (SHR) rats, of bilateral adrenal demedullation and subcutaneous slow-release implants containing adrenaline, salbutamol and procaterol were studied. Systolic blood pressure (SBP) measurements were made, using an indirect tail-cuff method in groups of male SHR rats aged from three to 14 weeks, which were sham-operated or bilaterally adrenal demedullated under ether anaesthesia at four weeks of age. Bilateral adrenal demedullation attenuated the development of a raised blood pressure, while slow-release implants of adrenaline restored the development of hypertension in adrenal demedullated rats and this effect was abolished by concomitant treatment with the beta 2-adrenoceptor antagonist [C] 118551. Implants of salbutamol and procaterol were also effective in restoring hypertension development in adrenal demedullated rats. It is suggested that adrenaline exerts its pro-hypertensive effects via an action at beta 2-adrenoceptors; possibly those located prejunctionally which are involved in the facilitation of sympathetic neurotransmission.

Effect of adrenal demedullation and adrenaline on hypertension development and vascular reactivity in young spontaneously hypertensive rats.

1. The effects of adrenal demedullation and subcutaneous depots of adrenaline, on hypertension development and vascular reactivity, were determined in young spontaneously hypertensive rats (SHRs). 2. Plasma adrenaline, but not noradrenaline, was significantly lower in all 16-week-old demedullated SHRs, irrespective of the time of demedullation. However, hypertension development was attenuated only in SHRs demedullated at 6 weeks of age or younger. 3. Adrenaline depots restored hypertension development in SHRs demedullated at 4 weeks, irrespective of the time of depot implantation, but were without effect in sham-operated rats. 4. Pressor responses to exogenous noradrenaline, in isolated perfused mesenteric arteries from 16-week-old demedullated and sham-operated SHRs and those treated with adrenaline depots, did not differ significantly. Maximal neurogenic pressor responses were, however, reduced in mesenteries from all demedullated rats, including those treated with depot adrenaline. Adding adrenaline to the perfusate facilitated neurogenic responses only. 5. Thus, the adrenal medulla appears to be involved in modulating sympathetic neurogenic vasoconstriction. The nature of this sympathoadrenal interaction and its role in the early development of hypertension in the SHR is unclear and is not explicable simply in terms of a facilitation of sympathetic neurogenic responses by adrenaline.

Kappa-opioid agonist induced diuresis. Is it mediated by a blood-borne factor of adrenomedullary origin?

Intravenous administration of the kappa-opioid agonists U50488H, tifluadom, and ethylketocyclazocine induced a characteristic diuresis in conscious, intact, saline-loaded rats. Naloxone pretreatment antagonized U50488H-induced diuresis. The diuretic response to the kappa-opioid agonists was significantly attenuated in adrenal demedullated rats. However, basal urine output, the diuretic response to furosemide, and the antidiuretic response to the mu-opioid agonist buprenorphine were unaffected. Transfusion studies established that 1 mL of blood, from intact donor rats treated with U50488H, induced a diuretic response when administered to intact or demedullated recipient rats, whether or not the recipients had been pretreated with naloxone. However, blood from demedullated rats treated with U50488H was unable to induce diuresis in intact or demedullated recipients. The results indicate that kappa-opioid agonist induced diuresis appears to be mediated by a nonopioid blood-borne "diuretic factor" of adrenomedullary origin and that this factor might be responsible for the dependence of the diuretic response upon an intact and functional adrenal medulla in conscious rats.

Cardiovascular changes in anaesthetised rats after the intra-hypothalamic administration of adrenaline.

Bilateral injections of adrenaline (0.01-10.0 microgram) into the anterior hypothalamic (AH) region, in urethane-anaesthetised spontaneous hypertensive (SH) rats, elicited dose-dependent falls in blood pressure and heart rate. The bradycardia was immediately in onset while the hypotension was preceded by a short-lasting rise in blood pressure. Bilateral injections of adrenaline into the anterior preoptic area (POA) and areas surrounding the AH had little or no effect on blood pressure and heart rate, while injections into the posterior hypothalamus (PH) induced tachycardia and hypertension followed by a smaller fall in blood pressure. Pretreatment with dl-propranolol (25-100 microgram bilat. AH) appeared to potentiate the hypotension induced by adrenaline (1 microgram bilat. AH) in a dose-dependent manner, but did not affect the falls in heart rate. On the other hand, pretreatment with metoprolol (25-100 microgram bilat. AH) effected a dose-dependent antagonism of the adrenaline-induced hypotension and bradycardia. Pretreatment with 1-propranolol (25 microgram bilat. AH) also antagonised the adrenaline-induced cardiovascular depressor effects, while pretreatment with d-propranolol (25 microgram bilat. AH) abolished the initial hypertensive effect. Pretreatment with piperoxan (25-100 microgram bilat. AH) antagonised adrenaline (1 microgram bilat. AH) induced hypotension and bradycardia only at the highest dose used. The results give further support to the concept that hypothalamic adrenaline receptors may be involved in the central regulation of blood pressure and heart rate. Furthermore, while an involvement of hypothalamic alpha-adrenoceptors cannot be ruled out, it is suggested that hypothalamic beta-adrenoceptors may be involved in mediating the cardiovascular depressor effects of adrenaline injected into the AH.

Adrenaline and the development of spontaneous hypertension in rats.

The effects of chronic alterations in plasma adrenaline levels, on the development of a raised blood pressure in young spontaneously hypertensive (SHR) rats, have been investigated. Bilateral adrenal demedullation (at 4 weeks) significantly reduced plasma adrenaline levels and attenuated the development of hypertension. Pressor responses to phenylephrine (0.3-10 micrograms, i.v.), measured in the pithed animals 11 weeks after demedullation, were unaffected although neurogenic pressor responses were significantly reduced. Subcutaneous implants of procaterol and salbutamol (0.005 and 0.165 mumol/kg, respectively) restored hypertension development in demedullated rats and significantly enhanced neurogenic pressor responses, while responses to i.v. phenylephrine remained unaltered, in the pithed rats. Implants of adrenaline (0.165 and 0.5 mumol/kg, s.c.) also restored hypertension development. However, pressor responses to phenylephrine were reduced and neurogenic responses only slightly enhanced when compared to those obtained in untreated pithed demedullated rats. The pro-hypertensive effect of adrenaline (0.5 mumol/kg, s.c.) was abolished by treatment with the beta 2-adrenoreceptor selective antagonist ICI 118551 (25 mg/kg/day, p.o.). In the subsequently pithed rats, neurogenic pressor responses were slightly reduced when compared to those in animals treated with adrenaline alone. In control demedullated rats, ICI 118551 had no effect on blood pressure nor on the neurogenic and phenylephrine-induced pressor responses. However, in sham-operated animals, ICI 118551 attenuated the development of hypertension and significantly reduced neurogenic pressor responses in the subsequently pithed rats. Responses to phenylephrine remained unaltered. The results support the suggestion that a beta 2-adrenoreceptor-mediated facilitation of sympathetic neurotransmission may be involved in mediating the pro-hypertensive effects of circulating adrenaline in the SHR rat.

Validation of indirect systolic blood pressure measurement in ether anaesthetised rats.

Systolic blood pressures and plasma catecholamine concentrations were determined in conscious and ether-anaesthetised male spontaneously hypertensive (SHR), Wistar-Kyoto normotensive (WKY/N), Wistar (W) and Sprague-Dawley (SD) rats. When compared to conscious animals, plasma noradrenaline concentrations were significantly higher in all four strains of ether-anaesthetised rats. Only the SHR rats did not show an elevation in plasma adrenaline concentration when anaesthetised with ether. A good correlation was obtained between systolic blood pressures measured indirectly, using a tail-cuff method with two different sensors, in ether-anaesthetised rats and pressures measured directly, via an indwelling carotid catheter, in conscious unrestrained animals. It is concluded that, while ether anaesthesia elevates plasma catecholamine concentrations in rats, the increase in plasma catecholamines has little, if any, effect on indirect systolic blood pressure measurements.

Beta-adrenoreceptor-mediated modulation of vasoconstriction in rat isolated perfused mesenteric arteries.

Pressor responses to bolus injections of noradrenaline (NA) were studied, in the isolated perfused rat mesenteric arterial bed, in the presence of beta-adrenoreceptor agonists and antagonists, in an attempt to identify a possible beta-adrenoreceptor mediated modulation of catecholamine-induced vasoconstrictor effects. NA-induced responses were potentiated in the presence of timolol and (-)propranolol and suppressed in the presence of (-)isoprenaline; (+)isoprenaline was less effective against the NA-induced responses. Timolol attenuated the effects of (-)isoprenaline on NA-induced responses but not those of the stereoisomer (+)isoprenaline. It is concluded that the NA-induced pressor effect in the rat mesenteric vasculature is the net result of vasoconstrictor alpha- and vasodilator beta-adrenoreceptor activation and that the interaction of the two opposing adrenoreceptor-mediated effects represents a 'physiological antagonism'.