Evidence for selective regulation of the phosphorylation of myocyte proteins by isoproterenol and prostaglandin E1.

Both isoproterenol and prostaglandin E1 increased the activation state of cyclic AMP-dependent protein kinase in cultured myocytes; however, only isoproterenol enhanced phosphorylase activity and contractile state. Following the incubation of intact myocytes with 32PO3-(4), 32 phosphoproteins were resolved from total cellular proteins by electrophoresis in sodium dodecyl sulfate polyacrylamide gels followed by autoradiography. Isoproterenol stimulated 32PO3-(4) incorporation into 16 proteins, including 2 phosphoproteins not observed under control conditions. By contrast, prostaglandin E1 neither caused a measurable change in the protein phosphorylation pattern nor interfered with isoproterenol's capacity to do so. Isoproterenol stimulated myocyte protein phosphorylation in either the presence or absence of extracellular Ca2+. The results suggest that the regulation of protein phosphorylation following adenylate cyclase stimulation is: (1) an agonist-specific process and not due solely to a random accumulation of intracellular cycle AMP and activation of protein kinase; (2) the Ca2+ mobilization component of beta-receptor activation does not account for the paradoxical effects of isoproterenol and prostaglandin E1; (3) activation of cyclic AMP-dependent protein kinase does not always result in an enhancement of protein phosphorylation.

Phosphorylation of ventricular sarcolemmal membranes does not alter binding properties of nitrendipine.

Isoproterenol increased contractility in isolated cat papillary muscles 2-fold with an EC50 of 6.3 X 10(-8) M. Nifedipine (3 X 10(-7) M) reduced contractility in control muscles by 43%; however, inotropic state was restored by isoproterenol with a comparable EC50 of 5 X 10(-8) M. To test the hypothesis that this effect might result from cAMP-dependent phosphorylation of a Ca2+ channel-associated protein, [3H]nitrendipine binding was used to probe the high-affinity 1,4-dihydropyridine site in either phosphorylated or dephosphorylated sarcolemmal vesicles. Kd and Bmax values for binding to phosphorylated sarcolemmal vesicles (0.14 +/- 0.027 nM and 479 +/- 62 fmol/mg protein, respectively) were not significantly different from control values P greater than 0.4). Similarly, dephosphorylation of sarcolemmal vesicles did not alter binding parameters. These data demonstrate that phosphorylation of sarcolemmal vesicles neither alters the binding affinity for [3H]nitrendipine nor promotes an interconversion of dihydropyridine-binding sites from high to low affinity or vice versa. While phosphorylation may regulate the slow Ca2+ channel, this is not reflected as changes in [3H]nitrendipine-binding parameters determined in vitro. Furthermore, the cyclic AMP-dependent phosphorylation state of sarcolemmal proteins does not appear to account for wide variations (more than 100-fold) between Kd values from binding studies and IC50 values determined in pharmacological investigations.

Cardiovascular effect and stimulus-dependent inhibition of superoxide generation from human neutrophils by tibenelast, 5,6-diethoxybenzo(b)thiophene-2-carboxylic acid, sodium salt (LY186655).

Tibenelast (LY186655), 5,6,-diethoxybenzo(b)thiophene-2-carboxylic acid, sodium salt, is an orally active anti-anaphylactic compound in guinea pigs, and has been shown to prevent bronchospasm in moderately severe asthmatic patients. Pharmacological studies with tibenelast demonstrated that it is a selective phosphodiesterase (PDE) inhibitor in that it is moderately active against the lung and stomach enzyme while being a very weak inhibitor of the heart enzyme. The compound was without cardiovascular effects at anti-anaphylactic doses. In contrast to theophylline, tibenelast did not have a direct inotropic effect in the cat papillary muscle system. The concentration that inhibited 50% of the enzymatic activity (IC50) for tibenelast was 20- to 30-fold lower for neutrophil PDE than for PDE of other tissues. It was 100 times more potent than aminophylline in inhibiting superoxide generation from platelet-activating factor (PAF)-primed polymorphonuclear leukocytes (PMNL) challenged with chemotactic factor, N-formyl-methionyl-leucyl-phenylalanine. However, tibenelast was less effective in the tumor necrosis factor-primed system, and did not inhibit superoxide generation during phagocytosis or when other soluble stimuli, such as phorbo-12-myristate-13-acetate or the calcium ionophore A23187, were used. Furthermore, tibenelast did not inhibit enzymes involved in arachidonic acid metabolism. These results suggest that tibenelast probably inhibits superoxide release from PMNL via a selective inhibition on PDE.

Ryanodine and an iodinated analog: doxorubicin effects on binding and Ca2+ accumulation in cardiac sarcoplasmic reticulum.

An 125I-iodinated ryanodine analog, modified by attaching an iodo-Cbz-beta-alanyl group to the C10eq hydroxy of ryanodine (iodo-carbobenzyloxy-beta-alanyl-ryanodine), binds to cardiac sarcoplasmic reticulum Ca2+ release channels with equal affinity as [3H]ryanodine. In the present study, both iodo-Cbz-beta-alanyl-ryanodine and ryanodine bound to canine cardiac microsomal membrane preparations in a Ca2+ dependent manner. At 10 microM free Ca2+ doxorubicin increased specific binding of both ligands, with doxorubicin concentrations of 4.06 +/- 0.44 and 6.22 +/- 1.31 microM inducing 50% maximal enhancement of binding for ryanodine and iodo-Cbz-beta-alanyl-ryanodine, respectively. Effects of ryanodine and iodo-Cbz-beta-alanyl-ryanodine +/- doxorubicin in vitro on cardiac sarcoplasmic reticulum Ca2+ release were compared indirectly by determining Ca2+ accumulation in cardiac microsomal vesicles loaded with 45Ca2+. In the absence of oxalate, neither ryanodine nor iodo-Cbz-beta-alanyl-ryanodine (10 microM) decreased net Ca2+ uptake, whereas doxorubicin reduced Ca2+ accumulation 20 +/- 2%. In the presence of oxalate and 0.4 microM free Ca2+ ("low"), both ryanodine and iodo-Cbz-beta-alanyl-ryanodine modestly decreased (by 19% and 17% at 10 nM, respectively) maximum Ca2+ accumulation. Increasing concentrations of ryanodine (100 nM-100 microM) and iodo-Cbz-beta-alanyl-ryanodine (100 nM-30 microM) had no greater effect, but 100 microM iodo-Cbz-beta-alanyl- ryanodine decreased net Ca2+ uptake 57 +/- 3%. Doxorubicin (30 microM) alone reduced Ca2+ uptake 36%; its effects with 1 nM-10 microM ryanodine or 1 nM-100 microM iodo-Cbz-beta-alanyl-ryanodine were additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac inotropic responses to calcium and forskolin are not altered by prolonged isoproterenol infusion.

Effects of prolonged isoproterenol infusion upon the density of cardiac calcium channels, calcium-mediated contractile responses, and the ability of forskolin to enhance tension development and cyclic AMP accumulation were studied in ventricular muscle preparations from Sprague-Dawley rats. Isoproterenol infusion (400 micrograms/kg per h s.c., 4 days) significantly decreased calcium channel density (Bmax) in cardiac microsomal membranes as quantified by a 32% decrease in specific [3H]nitrendipine binding sites; binding affinity (KD) was unchanged. A 57% decrease of beta-adrenoceptors confirmed homologous down regulation. To examine functional effects of decreased [3H]nitrendipine binding sites, responses to calcium, BAY K8644 and nifedipine were determined in isolated right ventricular strips. Significant decreases in basal developed tension were observed in muscles from isoproterenol-infused rats. However, concentration-dependent increases in contractility in response to CaCl2 or BAY K8644 were comparable, and the negative inotropic effect of nifedipine was unchanged. Whereas isoproterenol infusion was associated with significantly decreased basal cardiac cyclic AMP concentrations, exposure of ventricular strips from either vehicle- or isoproterenol-infused rats to 10 microM forskolin resulted in comparable increases in cyclic AMP and in developed tension. Cumulative, submaximal concentrations of forskolin also produced similar increases in contractility with maximum responses in ventricular strips from vehicle-infused animals attained at 4.4 microM forskolin. Higher concentrations resulted in automaticity. By contrast, ventricle from isoproterenol-infused animals responded to 14.4 microM forskolin with maximal increases in force of contraction.

Role of the alpha agonist activity of dobutamine in mediating cardiac output: effects of prolonged isoproterenol infusion.

Hemodynamic activities of dobutamine enantiomers were studied in either control rats or those infused with isoproterenol (400 micrograms/kg/hr) for 4 days. In control animals prazosin attenuated the effects of (+/-)-dobutamine on cardiac output by approximately 50%; remaining activity was blocked by propranolol. After isoproterenol infusion (+/-)-dobutamine was less efficacious and the blocking effects of prazosin were greater than 90%. Isoproterenol infusion had no effect on (-)-dobutamine-mediated (alpha-1 adrenoceptor agonist) increases in cardiac output and these actions were blocked by prazosin. By contrast, compared to (+/-)- and (-)-dobutamine, effects of (+)-dobutamine (beta adrenoceptor agonist) on cardiac output were modest, not altered by prazosin and were blocked by propranolol; (+)-dobutamine was inactive after isoproterenol infusion. (-)-Dobutamine increased systemic vascular resistance in both control and isoproterenol infused rats, whereas (+)-dobutamine was inactive. (+/-)-Dobutamine increased systemic vascular resistance only in isoproterenol-infused rats. All increases in systemic vascular resistance were blocked by prazosin. Neither (+/-)- nor (+)-dobutamine significantly altered stroke volume. By contrast, (-)-dobutamine resulted in prazosin-sensitive increases in stroke volume in both control and isoproterenol-infused rats. In control animals, (+/-)-, (+)- and (-)-dobutamine increased heart rate in a dose-dependent manner; chronotropic effects of (-)-dobutamine were less than those of either (+/-)- or (+)-dobutamine. Chronotropic effects were not demonstrable in isoproterenol-infused animals. These data support the notion that in control rats cardiac output may be increased by either alpha or beta adrenoceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of beta adrenoceptor down-regulation on the cardiovascular responses to the stereoisomers of dobutamine.

Effects of prolonged in vivo infusion of either saline (control) or isoproterenol (beta adrenoceptor desensitization) on acute cardiovascular responses to (+) (beta agonist)-, (-) (alpha agonist)- and (+/-)-dobutamine were studied in pithed rats. Each form of dobutamine resulted in comparable dose-dependent increases in maximum left ventricular dP/dt (LVdP/dtmax) in control animals. Effects of (+)-dobutamine were blocked by propranolol whereas those of l-dobutamine were sensitive to prazosin; both alpha and beta antagonists were required to block the inotropic effects of the racemic mixture. Contractile responses to (+)- and (+/-)-dobutamine were accompanied by tachycardia (characteristic of beta adrenoceptor stimulation) whereas (-)-dobutamine enhanced LVdP/dtmax without altering heart rate (characteristic of alpha adrenoceptor stimulation). Isoproterenol infusion resulted in a pronounced desensitization to the inotropic effects (LVdP/dtmax) of (+/-)- and (+)-dobutamine. Ed30 values for (+/-)- and (+)-dobutamine were increased by approximately 15- and 50-fold, respectively, and maximal responses to both drugs were severely attenuated. Prazosin further blunted remaining inotropic responses to (+/-)-dobutamine and propranolol resulted in a complete block. Responses to (+)-dobutamine were only sensitive to propranolol. Attenuation of heart rate responses paralleled those observed for LVdP/dtmax. By contrast, the inotropic effects of (-)-dobutamine in either control or desensitized rats were both qualitatively and quantitatively comparable; responses were blocked by the alpha-1 antagonist, prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and biochemical properties of an 125I-labelled ryanodine derivative.

The synthesis of a novel radioiodinated ryanodine-O10eq-N-acylamino acylate along with biological data are reported. The affinity of the iodinated product, 7, was comparable to ryanodine, 7.97 nM and 6.47 nM, respectively. Conversion of the non-radioactive iodinated ryanodine analog to the [125I] isotope was accomplished by conversion of 7 to the trimethyltin derivative followed by [125I] exchange using chloramine-T in organic solvent. The radioiodinated ryanodine analog, 9, bound to cardiac membrane preparations in a protein dependent and saturable manner indicating that this analog may represent a useful new tool for the study of ryanodine receptors and that modifications about the C-10 hydroxy group of ryanodine may be carried out without loss in biological activity.

Contractility and protein phosphorylation in cardiomyocytes: effects of isoproterenol and AR-L57.

The cardiotonic drugs AR-L57 [2-(2,4-dimethoxyphenyl)-1H-imidazo(4,5b)-pyridine] and isoproterenol stimulated contractility in cultured heart cells in concentration-dependent manners; only the effects of isoproterenol were blocked by propranolol. Isoproterenol, but not AR-L57, enhanced the phosphorylation state of seven protein bands [relative molecular weights (MrS) 155,000, 96,000, 27,000, 24,000, 20,000, 16,000, 12,000] and resulted in the dephosphorylation of one protein band (Mr 21,000). Also, only isoproterenol increased the activation states of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase and glycogen phosphorylase. The eight protein bands resolved by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and detected by autoradiography were altered by isoproterenol in time- and concentration-dependent manners. The 24,000-Mr protein substrate phosphorylated in response to isoproterenol was converted to a 12,000-Mr species by heating in the presence of SDS prior to electrophoresis, suggesting that the two substrates were in fact identical proteins. A comparison of the 2-min responses to varying concentrations of isoproterenol resulted in excellent correlations between the phosphorylation states of individual protein bands and contractility. This was true even for the 21,000-Mr species that was dephosphorylated. However, only the 27,000-, 24-12,000-, and 16,000-Mr substrates were phosphorylated rapidly enough to be associated with the onset of the inotropic response. Cultured myocytes are an important feature of these studies as they are 84% pure ventricular cells that remain 100% viable throughout an experiment. Because this system is suitable for biochemical measurements and the effects of agents on heart cell contractility can be determined, it is possible to correlate changes in biochemical parameters with alterations in physiological state.

Effects of S-dobutamine on ischemic myocardium caused by coronary artery narrowing.

Cardiovascular effects of S-dobutamine were compared with effects of vehicle and other catecholamines in dogs during and after 3 days of approximately 90% ligation of the left anterior descending coronary artery (LAD). Twenty-four hours after LAD ligation, dogs infused with S-dobutamine (2.5 micrograms/kg/min intravenously, i.v.) maintained systolic blood pressure (SBP 149 +/- 6 mm Hg), diastolic blood pressure (DBP 100 +/- 6 mm Hg), and aortic dP/dt60 (2.8 +/- 0.2 s-1), with no significant changes from preligation values. In comparison, saline-treated dogs showed decreases in arterial BP and contractility: SBP 121 +/- 4 mm Hg; DBP 85 +/- 3 mm Hg; and aortic dP/dt60 was 1.9 +/- 0.1 s-1. S-Dobutamine-infused dogs had a heart rate (HR) of 148 +/- 5 beats/min with 44 +/- 14 beats/min premature ventricular contractions (PVCs), whereas dogs infused with saline, R-dobutamine, dopamine, norepinephrine (NE), or isoproterenol (ISO) all displayed a significantly greater number of PVCs at 24 h. Myocardial necrosis was limited by S-dobutamine treatment (2.5 micrograms/kg/min i.v. for 54 h). As demonstrated by histologic examination, S-dobutamine ameliorated the effects of ischemia as compared with vehicle, R-dobutamine, dopamine, hexamethonium, NE, or ISO. Myocardial tissue electrolytes, quantified 72 h after LAD ligation, were maintained by S-dobutamine-infused dogs in all sections of left ventricle (LV); but in saline-treated dogs, Ca2+ increased eightfold, Na+ increased twofold, and both K+ and Mg2+ decreased 50% in tissue "at risk" as compared with tissues "not at risk." Coronary nutrient blood flow (CNBF) to myocardial capillary vessels was calculated by radiolabeled microspheres 2 h after LAD ligation. As compared with CNBF in untreated hearts, endocardial CNBF in hearts receiving S-dobutamine (5 micrograms/kg/min i.v.) increased from 26 +/- 8 to 49 +/- 15 ml/min/100 g in tissue at risk, from 102 +/- 26 to 217 +/- 50 in "border zone," and from 133 +/- 13 to 215 +/- 41 in tissue not at risk. CNBF values in animals receiving vehicle infusion were not significantly different from CNBF values measured after ligation only. The S-enantiomer of dobutamine, infused in dogs for 54 h after coronary artery ligation, maintained cardiac performance, electrolyte balance, and myocardial cellular viability and reduced incidences of arrhythmias through its ability to increase CNBF without increasing HR.

Roles for Ca++ and cyclic AMP in mediating the cardiotonic actions of isomazole (LY175326).

The contractile state of cat papillary muscles was increased by isomazole in a concentration-dependent manner; inotropic effects of the drug were not altered by either prazosin, propranolol or cimetidine. Isomazole inhibited the peak III isozyme of dog heart phosphodiesterase with an IC50 of 100 microM; effects on isozymes I and II were less pronounced. In cat papillary muscles, carbachol (10(-5) M) shifted the relationship between contractility and concentration of isomazole to the right. These data suggest cyclic AMP (cAMP) is involved in the actions of isomazole. In order to assess the relative effects of isomazole on intracellular cAMP and Ca++, cAMP-dependent protein kinase and glycogen phosphorylase, respectively, were used as reporters of these two second messengers. The source of enzymes was either cultured cardiomyocytes or right ventricular biopsies obtained from anesthetized dogs. In the latter case, biopsies were obtained after i.v. administration of isomazole; the pure beta agonist, isoproterenol, was included for comparative purposes. A submaximal inotropic dose of isomazole (0.1 mg/kg i.v.) in dogs resulted in a pronounced increase in contractility that was associated with a 3-fold increase in phosphorylase activity (0.15 +/- 0.01 to 0.46 +/- 0.06, -5'-AMP: +5'-AMP, P less than .05); the activation state of protein kinase was not altered. By contrast, a comparably effective inotropic dose of isoproterenol (0.1 microgram/kg) caused less than a 2-fold increase in phosphorylase activity (0.15 +/- 0.01 to 0.26 +/- 0.02, -5'-AMP: +5'-AMP, P less than .05) and this was associated with a significant increase in the protein kinase activity ratio (0.36 +/- 0.01 to 0.51 +/- 0.04, -cAMP: +cAMP, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of LY195115, a potent, orally active cardiotonic with a long duration of action.

Compound LY195115 is a novel cardiotonic with both inotropic and vasodilator activities. In cat papillary muscles, LY195115 increased contractility in a concentration-dependent manner; its actions were not blocked by either prazosin or propranolol. An intravenous dose of 7.0 micrograms/kg LY195115 resulted in a 50% increase in contractility in anesthetized dogs; comparable inotropic responses were observed in anesthetized cats receiving 10 micrograms/kg i.v. These doses of LY195115 increased heart rates of both dogs and cats by less than 10%. Oral administration of 25 micrograms/kg to conscious dogs was associated with a selective inotropic response that was maximal at 3 h and maintained in excess of 23 h. This effect was not accompanied by gross behavioral changes or emesis. The hemodynamic profile of LY195115 was evaluated in anesthetized beagle dogs. A 60-min infusion of 1.0 microgram/kg/min LY195115 followed by a 5-min infusion of 10 micrograms/kg/min resulted in dose-dependent increases in contractility (LV dP/dt60) and heart rate; doses that increased LV dP/dt60 by 50% increased heart rate by less than 10%. Doses of greater than 5.0 micrograms/kg decreased left ventricular end-diastolic pressure and systemic vascular resistance; mean arterial blood pressure and cardiac output were unchanged. Estimated myocardial oxygen consumption (heart rate times either systolic or mean arterial blood pressure) was not altered by doses as high as 110 micrograms/kg. This balance of inotropic/vasodilator activities may provide a means of improving cardiac function while maintaining myocardial oxygen supply/demand.

Effects of S-dobutamine on venous blood return and organ nutrient blood flow.

The selective contractile effects of s-dobutamine were studied in vitro in selected canine arteries and vein preparations; propranolol was included to block potential beta-mediated vasodilation. These in vitro data were expanded by quantifying the in vivo effects of s-dobutamine on venous blood return and redistribution of regional nutrient blood flow (NBF) and non-nutrient blood flow (non-NBF) in anesthetized dogs. In in vitro studies with isolated canine arteries and veins, s-dobutamine exhibited vein-selective constriction. At maximally efficacious concentrations of agonist, contractions of carotid, coronary, and femoral arteries in response to s-dobutamine were only 7, 25 and 45% as great as those elicited by norepinephrine (NE). Similarly, in jugular vein, s-dobutamine-mediated contractions were 55% as great as those obtained in response to NE. Coronary and femoral arteries precontracted with NE were relaxed in a dose-related manner by increasing concentrations of s-dobutamine. Effects of NE and s-dobutamine on venous blood return (VR) were compared in dogs. s-Dobutamine increased VR by 49 +/- 10 ml, whereas NE increased VR by 14 +/- 6 ml during 5-min infusion. s-Dobutamine significantly increased coronary NBF in left ventricular (LV) endocardium from 115 +/- 10 to 194 +/- 13 and 263 +/- 9 ml/min/100 g at doses of 10 and 20 micrograms/kg/min, respectively. In addition, LV epicardium flow was increased from 87 +/- 8 to 189 +/- 15 and 262 +/- 11 ml/min/100 g at 10 and 20 micrograms/kg/min, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of LY175326: a potent cardiotonic agent with vasodilator activities.

Compound LY175326 is one of a series of novel cardiovascular agents with both inotropic and vasodilator activities. In cat papillary muscles, LY175326 increased contractility in a concentration-dependent manner; these actions were not blocked by prazosin, propranolol or cimetidine. Inotropic responses were observed in unpaced, perfused guinea-pig hearts and these effects were associated with modest increases in heart rate and coronary flow. An i.v. dose of 0.1 mg/kg of LY175326 caused 54 and 95% increases in contractility in either the anesthetized cat or dog, respectively; corresponding heart rates were increased by less than 10%. Oral administration of 0.5 mg/kg to dogs was associated with an inotropic response that was maximal between 60 and 90 min and lasted in excess of 3 hr. These effects were not accompanied by increases in heart rate, gross behavioral changes or emesis. The pharmacology of LY175326 was evaluated in a propranolol-induced heart failure model using anesthetized beagle dogs. A bolus injection of 0.15 mg/kg of LY175326 followed by an infusion of 0.4 mg/kg/hr reversed the hemodynamic symptoms of heart failure by increasing left ventricular dP/dt60, cardiac output and stroke volume and reducing left atrial filling pressure and vascular resistance; heart rate was unchanged and calculated myocardial oxygen consumption was reduced. This balance of inotropic:vasodilator activities may provide a means of improving cardiac function while maintaining the myocardial oxygen supply:demand.

Interaction of 5-HETE, 12-HETE, 15-HETE and 5,12-diHETE at the human platelet thromboxane A2/prostaglandin H2 receptor.

A variety of lipoxygenase products such as 12- and 15-hydroxyeicosatetraenoic acid (12- and 15-HETE) inhibit thromboxane A2 (TXA2) mimetic induced human platelet aggregation in a stereoselective manner. The mechanism of this inhibition remains unclear. To determine if this inhibition is due to a receptor level interaction of the lipoxygenase products at the thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor, radioligand binding studies were performed using a new [125I]-labelled thromboxane mimetic [125I]BOP. The mono-HETES 5(S), 12(R), 12(S) and 15(S) inhibited binding of the radioligand to the TXA2/PGH2 receptor in washed human platelets with IC50 values of greater than 25, 0.73, 2.06 and 2.0 microM respectively. LTB4 and its positional isomer 5(S), 12(S)-diHETE were less potent with IC50 values greater than 10 microM for LTB4 and 9.38 microM for 5(S), 12(S)-diHETE. Thus, stereoselective inhibition of the binding of the radioligand was demonstrated between 12(R)- and 12(S)-HETE. These lipoxygenase products also inhibited IBOP (10nM) induced platelet aggregation in a concentration dependent fashion with a similar rank order of potency as that obtained in the competition binding assay. These results suggest that, at least in part, the platelet inhibitory properties of these HETEs may be mediated through their interaction at the TXA2/PGH2 receptor.

Effects of prolonged phenylephrine infusion on cardiac adrenoceptors and calcium channels.

Effects of prolonged in vivo infusion of phenylephrine upon receptor binding and cardiac contractility were studied in Sprague-Dawley rats. A 1-hr i.v. infusion of phenylephrine (3 mg/kg/hr) resulted in a sustained 50% increase in diastolic blood pressure and 5% increase in heart rate. Chronic (6-day) infusion (3 mg/kg/hr) utilizing Alzet mini-osmotic pumps maintained plasma concentrations of phenylephrine at 1.0 microgram/ml, depleted myocardial norepinephrine stores 8-fold and resulted in a modest cardiac hypertrophy. Density and affinity of myocardial adrenoceptors and calcium channels were quantified by analyzing saturation isotherms of radioligand binding. [3H]Prazosin, [3H]dihydroalprenolol and [3H]nitrendipine bound specifically and with high affinity to cardiac alpha-1 and beta adrenoceptors and calcium channels, respectively. As measured by Scatchard analyses, phenylephrine infusion significantly decreased the maximum number (Bmax) of specific [3H]prazosin binding sites by 39% (430 +/- 20 vs. 263 +/- 16 fmol/mg of protein; P less than .05). Chronic phenylephrine treatment also decreased the Bmax for [3H]dihydroalprenolol binding by 31% (124 +/- 3.3 vs. 86 +/- 6.6 fmol/mg of protein; P less than .05) and the Bmax for [3H]nitrendipine binding by 32% (342 +/- 8.8 vs. 235 +/- 6.7 fmol/mg of protein; P less than .05). Binding affinities (Kd) of [3H]prazosin, [3H]dihydroalprenolol and [3H]nitrendipine remained unchanged. Administration of vehicle alone or surgical manipulation due to osmotic pump implantation did not affect either the density or affinity of [3H]prazosin, [3H]dihydroalprenolol or [3H]nitrendipine binding. Contractile responses to phenylephrine were studied in isolated ventricular strips to determine the functional significance of alpha-1 adrenoceptor down-regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis for the cardiovascular activities of amrinone and AR-L57.

It has been suggested that amrinone and AR-L57 enhance cardiac contractility either by inhibiting phosphodiesterase activity or altering Ca++ homeostasis. Because these novel agents are potentially useful in the management of heart failure, it was of interest to more clearly define their mechanism(s) of action. Amrinone and AR-L57 caused concentration-dependent increases in the contractile states of either perfused guinea-pig hearts or cultured rat cardiomyocytes. To determine whether these actions might result from an increase in sarcolemmal Ca++ movement, the effects of these agents on Ca++ accumulation were studied in a simple system, dog erythrocytes. Both agents promoted erythrocyte Ca++ accumulation in time and concentration-dependent manners, effects that resulted primarily from increased Ca++ entry. However, because these effects were not measurable at inotropic drug concentrations and were apparent only after a 30-min incubation, they did not provide an explanation for the inotropic effects of these agents. Amrinone and AR-L57 inhibited dog heart phosphodiesterase activity (isozyme III) with EC50 values of 23 and 420 microM, respectively; however, only the inotropic responses to amrinone were attenuated by the muscarinic agonist, carbachol, thereby implying a cAMP (cyclic AMP)-dependent mechanism. In cultured ventricular cells, concentrations of amrinone (2 X 10(-4) M) and AR-L57 (3 X 10(-5) M) that caused maximal inotropic responses were associated with the activation of glycogen phosphorylase, but neither drug significantly increased the activation state of cAMP-dependent protein kinase. To further probe the effects of these drugs on intracellular cAMP and Ca++ metabolism, their effects on protein phosphorylation were studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of thromboxane A2/prostaglandin H2 binding sites in guinea pig cardiac membrane preparations.

Thromboxane A2 (TXA2) and prostaglandin H2 (PGH2) induce platelet aggregation and are potent vasoconstrictors, and they have been implicated in coronary vasospasm and myocardial infarction. The TXA2 mimetic [1S-(1 alpha, 2 beta (5Z), 3 alpha (1E,3S*), 4 alpha)]-7-[3-(3-hydroxy-4-(4'- iodophenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-5-h eptenoic acid (IBOP) was used to characterize binding to microsomal membrane preparations from saline-perfused guinea pig atria (GPA) and ventricles (GPV). [125I]IBOP bound to GPA and GPV in a protein-dependent and saturable manner, although total binding was two-fold greater and non-specific binding was proportionately less in GPA compared to GPV. Analysis of equilibrium binding data indicated one class of binding sites in both GPA and GPV with Kd values of 333 +/- 117 and 645 +/- 187 pM, respectively, which were in close agreement with kinetically determined Kd values of 226 and 882 pM, respectively. Bmax values of GPA and GPV of 57 +/- 5.6 and 24 +/- 4.3 fmol/mg protein were significantly different (P < 0.01). Ki values (from IC50s) were determined for various TXA2/PGH2 analogues and prostaglandins in competition binding assays with [125I]IBOP. The rank order for ability to inhibit binding in GPA was U46619 = SQ29548 > I-PTA-OH > PGF2 alpha = PGE2. In GPV, the rank order was U46619 = SQ29548 > PGF2 alpha = I-PTA-OH = PGE2. [125I]IBOP binding to GPA and GPV was completely displaced by the TXA2/PGH2 agonist U46619 and by the TXA2/PGH2 antagonist SQ29548.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C-alpha and -epsilon modulate connexin-43 phosphorylation in human heart.

We have previously demonstrated that protein kinase C (PKC)- alpha expression is significantly elevated in failing human left ventricle, with immunostaining showing increased PKC- alpha localization at the intercalated disks of cardiomyocytes. In the present study we sought to determine, in the failing heart, if PKC- alpha interacted with connexin-43 (Cx-43) both spatially and functionally, and to compare the association of PKC- alpha/Cx-43 with that of PKC- epsilon, a PKC isozyme that does not significantly increase in failing hearts. The possibility of a PKC- alpha or PKC- epsilon/Cx-43 association in non-failing hearts was also investigated. Co-immunoprecipitation of PKC- alpha or PKC- epsilon and Cx-43 in non-failing and failing left ventricle was achieved using antibodies to PKC- alpha or Cx-43. Confocal microscopy confirmed that PKC- alpha distribution within the cardiomyocyte included co-localization with connexin-43 in both failing and non-failing myocardium. In a similar manner, confocal imaging of PKC- epsilon showed cardiomyocyte distribution in both cytosol and membrane, and colocalization of PKC- epsilon with Cx-43. Recombinant PKC- alpha or - epsilon increased PKC activity significantly above endogenous levels in the co-immunoprecipitated Cx-43 complexes (P<0.05). However, phosphorylation of purified human Cx-43 (isolated from failing human left ventricle) by recombinant PKC- alpha or PKC- epsilon resulted in only PKC- epsilon mediated Cx-43 phosphorylation. Thus, in the human heart PKC- alpha, PKC- epsilon, and Cx-43 appear to form a closely associated complex. Whereas only PKC- epsilon directly phosphorylates Cx-43, both PKC isoforms result in increased phosphorylation within the Cx-43 co-immunoprecipitated complex.