Alpha-adrenergic stimulation of sarcolemmal protein phosphorylation and slow responses in intact myocardium

Phospholamban and troponin I are substrates for protein kinase C in vitro but not in intact beating guinea pig hearts

The incorporation of [32P]inorganic phosphate into membranous, myofibrillar, and cytosolic proteins was studied in Langendorff-perfused guinea pig hearts treated with phorbol 12-myristate 13-acetate (PMA) or 1,2-dioctanoylglycerol (D8G), which are potent activators of protein kinase C. Control hearts were perfused with an inactive phorbol ester (4 alpha-phorbol 12,13-didecanoate), which does not cause activation of protein kinase C. To ensure the blockade of different receptor systems, the perfusions were carried out in the presence of prazosin, propranolol, and atropine. Perfusion of hearts with either PMA (4 microM) or D8G (200 microM) was associated with a negative effect on left ventricular inotropy and relaxation. Examination of the 32P incorporation into various fractions revealed that there were no increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum, and troponin I and C protein in the myofibrils, although these proteins were found to be substrates for protein kinase C in vitro. However, in the same hearts, there were significant changes in the 32P incorporation into a 28-kDa cytosolic-protein. Examination of the activity levels of protein kinase C in hearts perfused with PMA indicated a redistribution of this activity from the cytosolic to the membrane fraction, suggesting the activation of the enzyme in vivo. These findings indicate that cardiac regulatory phosphoproteins, which may be phosphorylated by protein kinase C in vitro, are not substrates for protein kinase C in beating hearts perfused with phorbol esters or diacylglycerol analogues.

Function of myocardial alpha-adrenoceptors

In addition to beta-adrenoceptors (beta ARs), cardiac myocytes of animals and man possess alpha 1ARs, but not alpha 2ARs. Norepinephrine and epinephrine have a higher affinity for myocardial alpha 1ARs than for beta ARs. Unlike beta AR stimulation, myocardial alpha 1AR stimulation does not increase the slow inward current. The alpha 1AR-mediated positive inotropic effect seen in isolated heart preparations appears to involve increased Ca sensitivity of myofibrils and production of inositol triphosphate (IP3) and diacylglycerol (DAG), but the functions of IP3 and DAG are not clear. Myocardial alpha 1AR stimulation reduces rate of isolated atria and Purkinje fibers and lengthens refractory period and action potential duration. Hypoxia increases alpha 1AR density in cardiomyocytes. alpha 1AR-mediated arrhythmias occur in isolated Purkinje fibers during hypoxia, following infarction, and in the presence of Ba2+ or high Ca2+. In animals, coronary artery occlusion and/or reperfusion increase myocardial alpha 1AR density and responsiveness, and alpha AR blocking drugs attenuate arrhythmias. However, an antiarrhythmic effect of alpha AR blocking drugs mediated by action on coronary vascular alpha ARs cannot be excluded. Presently available drugs do not differentiate between myocardial and vascular alpha ARs and thus affect the coronary and systemic circulations and, indirectly, the heart. Additional myocardial alpha 1AR-mediated effects include production of cardiac hypertrophy, stimulation of glucose uptake and phosphofructokinase and cyclic AMP phosphodiesterase activity, and release of atrial natriuretic peptide.

Characterization of cytoplasmic and membrane-associated phosphatidylinositol 4,5-biphosphate phospholipase C activities in guinea pig ventricles

The phosphoinositide-specific phospholipase C (PLC) activity present in the soluble and sarcolemmal enriched membrane fraction from guinea pig hearts was characterized using phosphatidyl [3H]inositol 4,5-biphosphate (PIP2) or phosphatidyl [3H]inositol 4-monophosphate (PIP) as substrates. The PLC activities (cytosolic and membrane associated) were specific for polyphosphoinositides (PIP2 and PIP) since no other phospholipids were hydrolyzed at pH 7.0 under various ionic conditions. Both enzymic activities were Ca2(+)-dependent (half maximal activities were achieved around pCa 5.0). The pH, detergent (deoxycholate), divalent (Ca2+ and Mg2+), and monovalent (Na+ and K+) cation dependencies were very similar between the cytosolic and membrane-associated enzyme activities, using either PIP2 or PIP as substrate. Hydrolysis of the polyphosphoinositides was inhibited in the presence of phosphatidylethanolamine, phosphatidylserine, or phosphatidylcholine. Under optimal conditions (pH 7.0, 1 mM Ca2+, 2.5 mM Mg2+, 100 mM Na+ and 0.07% deoxycholate) the specific activities of the cytosolic and membrane-associated enzymes were 19.9 +/- 0.9 and 10.1 +/- 0.9 nmol/min/mg protein, respectively, using PIP2 as substrate. Under the same conditions these activities were 18.1 +/- 1.0 and 8.0 +/- 0.8 nmol/min/mg protein for the cytosolic and membrane fractions, respectively, using PIP as substrate. Based on the similarity of the characteristics of these two PLC enzyme activities, it is suggested that the cytosolic and membrane-associated enzyme forms may be closely related.

Changes in phosphoinositide turnover in isolated guinea pig hearts stimulated with isoproterenol

The incorporation of 32Pi into phospholamban, troponin I, phosphatidylinositols, and inositol trisphosphates was studied in Langendorff-perfused guinea pig hearts stimulated with isoproterenol. Hearts were perfused with Krebs-Henseleit buffer containing [32P]Pk and freeze-clamped at different times during the positive inotropic response. Exposure of the hearts to 0.1 microM isoproterenol for up to 1 minute was associated with significant (up to threefold) increases in phospholamban and troponin I phosphorylation, but there was no significant increase in 32P incorporation into phospholipids. However, longer exposure (2 minutes or more) to isoproterenol was associated with increases in the degree of 32P labeling of phosphatidylinositols and phosphatidic acid. Examination of 32P labeling of inositol trisphosphates in the same hearts revealed that the radioactivity associated with these compounds decreased with time. The decreases were significant at times of exposure of 2 minutes or longer to beta-adrenergic stimulation. The tissue levels of the inositol 1,4,5-trisphosphate isoform were also measured in hearts perfused with isoproterenol for 3 minutes, and they were found to be significantly lower compared with values obtained in control hearts. The effects of isoproterenol on 32P incorporation into phospholipids and proteins were observed in the presence of prazosin, and they were completely abolished by the beta-receptor blocker propranolol. Examination of the phosphoinositide-specific phospholipase C activity in the perfused hearts revealed that isoproterenol stimulation was associated with a decrease in the membrane-associated enzymatic activity at physiological calcium concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms contributing to the arrhythmogenic influences of alpha 1-adrenergic stimulation in the ischemic heart

The majority of deaths associated with ischemic heart disease occur suddenly because of disturbances in cardiac rhythm culminating in ventricular fibrillation. Past research has focused on elucidating the biochemical membrane mechanisms responsible for the adverse electrophysiologic alterations in the ischemic heart, with major emphasis on the influence of adrenergic neural factors. It has been demonstrated that both alpha 1-and beta-adrenergic mechanisms contribute to arrhythmogenesis in the ischemic heart. In the normal heart, alpha 1-adrenergic input has very little effect on electrophysiologic indices. However, during early ischemia and reperfusion, enhanced alpha 1-adrenergic responsivity associated with a twofold reversible increase in alpha 1-adrenergic receptors in vivo has been demonstrated. Likewise, in a variety of species, alpha 1-adrenergic inhibition with prazosin markedly decreases the incidence of malignant ventricular arrhythmias associated with either myocardial ischemia or subsequent reperfusion. One major manifestation of alpha 1-adrenergic receptor activation during reperfusion of ischemic myocardium is an increase in intracellular calcium ion (Ca2+). It has been demonstrated that reperfusion of ischemic myocardium increases intracellular Ca2+ in reversibly injured tissue, and that the gain in intracellular Ca2+ is prevented by alpha 1-adrenergic inhibition with hydroxyphenylethyl aminomethyl tetralone, even when administered just prior to reperfusion. Subsequently, it was demonstrated that the alpha 1-adrenergic-induced increase in mitochondrial Ca2+ contributes to the decline in mitochondrial function. These findings suggest that even single-dose intervention with alpha 1-adrenergic inhibitors may improve markedly the functional recovery and extent of ultimate necrosis in humans after coronary thrombolysis. To investigate the mechanisms responsible for the increase in alpha 1-adrenergic receptors during ischemia, we used isolated adult canine ventricular myocytes exposed to hypoxia. Thirty minutes of hypoxia at 25 degrees C or 10 minutes of hypoxia at 37 degrees C resulted in a threefold reversible increase in the density of surface alpha 1-adrenergic receptors and a threefold increase in the cellular content of long-chain acylcarnitines. Inhibition of carnitine acyltransferase I abolished not only the accumulation of long-chain acylcarnitines during hypoxia but also the increase in alpha 1-adrenergic receptors. Exposure of normoxic myocytes to exogenous long-chain acylcarnitines (1 mumol/liter) for 10 minutes also increased alpha 1-adrenergic receptor number. These findings indicate that the sarcolemmal accumulation of long-cha

Myocardial stretch stimulates phosphatidylinositol turnover

The mammalian myocardium responds to stretch by increasing both contractility and the release of atrial natriuretic peptide. These effects are observed in isolated perfused heart preparations as well as in vivo. That atrial natriuretic peptide release can be stimulated by activation of the phosphatidylinositol turnover pathway suggests a possible mechanism by which stretch might activate a biological response. Accordingly, experiments were performed to examine the effect of dilatation of the right atrium on the phosphatidylinositol turnover pathway measured in isolated perfused hearts. Dilatation of the right atrium caused a stimulation of the phosphatidylinositol turnover pathway as measured by an increase in the accumulation of inositol phosphates. In right atria, increases were detected after 1 minute of dilatation, and maximal increases were observed after 10 minutes. Dilatation for 10 minutes caused an increase in inositol monophosphate, inositol bisphosphate, and inositol trisphosphate from 23.3 +/- 0.9, 15.4 +/- 0.4, and 9.5 +/- 0.3 cpm/mg tissue (mean +/- SEM, n = 7) to 74.6 +/- 2.3, 20.2 +/- 1.3, and 13.6 +/- 1.5 cpm/mg tissue (n = 8), respectively (p less than 0.01 for all inositol phosphates). Smaller increases were observed in the other chambers of the hearts. Perfusion with propranolol, prazosin, and atropine (all 1 microM) did not alter the inositol phosphate response to dilatation, indicating that it was not secondary to release of norepinephrine or acetylcholine. Dilatation of the right ventricle also caused a stimulation of inositol phosphate accumulation, but this was lower than after dilatation of the right atrium. These results show that the myocardium can respond to dilatation by an activation of the phosphatidylinositol turnover pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

[Inositol trisphosphate, a new "second messenger" for positive inotropic effects on the heart?]

Myocardial alpha 1-adrenoceptors mediate a positive inotropic effect and influence the inositol phosphate cycle. The receptor-stimulated, phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate (PIP2) results in the generation of two novel second messengers, inositol trisphosphate (IP3) and diacylglycerol (DG). This effect is concentration-dependent and precedes the increase in force of contraction. Recently, it has been shown that the alpha 1-adrenoceptor-mediated increase in IP3 and force of contraction exists in the human heart as well. Possible mechanisms for an inositol phosphate-mediated positive inotropic effect are: (i) release of Ca2+ from the sarcoplasmic reticulum, elicited by IP3, (ii) increase in Ca2+ sensitivity of the contractile proteins, elicited by IP3, inositol tetrakisphosphate (IP4) and/or DG, (iii) increase in slow Ca2+ inward current, elicited directly by IP4 and/or indirectly by DG through a phosphorylation of the protein kinase C substrate in the sarcolemma. In ventricular cardiac preparations muscarinic agonists have a weak positive inotropic effect, but in cardiac atrial preparations they have a negative inotropic effect. In both preparations, these different effects coincide with a concentration-dependent increase in IP3. Thus, the possible positive inotropic effect in atrial preparations is probably masked by an activation of a K+ outward current. The relationship between the inositol phosphate cycle and the positive inotropic effect is in some points still speculative because not all of the mechanisms discussed are well settled yet. However, the stimulation of myocardial phosphoinositide breakdown resulting in an increased IP3 may be involved in the mechanism(s) whereby alpha1-adrenergic and muscarinic receptor stimulation exert an increase in myocardial force of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbamoylcholine-induced accumulation of inositol mono-, bis-, tris- and tetrakisphosphates in isolated cardiac myocytes from adult rats

The effect of carbamoylcholine on the phosphoinositide cycle in isolated ventricular myocytes from adult rats was studied. Separation of the phosphoinositides by high-performance thin-layer chromatography showed a constant ratio of incorporation of myo-[2-3H]inositol into phosphatidylinositol, phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate of cultured cardiac myocytes after at least 2 h. Carbamoylcholine caused a dose-dependent and time-dependent accumulation of inositol mono-, bis- and trisphosphates, which was antagonized by atropine. Using anion-exchange HPLC the existence of inositol 1,4,5-trisphosphate, inositol 1,3,4-triphosphate and inositol 1,3,4,5-tetrakisphosphate was confirmed in rat ventricular myocytes. Inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate accumulated within 20 s, while inositol 1,3,4-trisphosphate, inositol 1,4-bisphosphate and inositol monophosphate increased within 5 min.

Adrenoceptor-mediated changes of action potential and force of contraction in human isolated ventricular heart muscle

1. The effects of alpha-adrenoceptor stimulation on the action potential and force of contraction were investigated in human isolated ventricular heart muscle and compared with those of beta-adrenoceptor stimulation. 2. The maximal stimulation by isoprenaline of beta-adrenoceptors produced large changes in the force of contraction, which were accompanied by moderate increases in the height of the action potential. The maximal inotropic effect produced by stimulation of alpha-adrenoceptors with phenylephrine, in the presence of propranolol (1 mumol 1(-1)) was much smaller (about 10% of that seen in response to beta-adrenoceptor stimulation), and no significant changes of the action potential configuration were observed. 3. The effects of noradrenaline and adrenaline on the force of contraction were not affected by prazosin. 4. It is concluded that the adrenoceptor-mediated changes of the force of contraction (in the presence of either noradrenaline or adrenaline) in the human ventricle are due virtually exclusively to the stimulation of beta-adrenoceptors.

Skeletal muscle sarcolemma proteins as targets for adenosine 3':5'-monophosphate-dependent and calcium-dependent protein kinases

The present study documents the existence in rat skeletal muscle plasma membrane (sarcolemma) of a distinct set of proteins, most of which represent unknown protein species, which can be phosphorylated in vitro by addition of cAMP-dependent or calcium-dependent protein kinases. Under the experimental conditions used, cAMP-regulated protein phosphorylation appeared to be the most important phosphorylation system in these membranes, followed by the calcium/phospholipid-regulated, and, with only a few substrates detected, the calcium/calmodulin-regulated systems. No specific substrate for cGMP-dependent protein kinase was found. In contrast, calcium/calmodulin-regulated protein phosphorylation was the most important in the sarcoplasmic reticulum fraction. Most of the cAMP-regulated and calcium/phospholipid-regulated sarcolemma phosphoproteins appeared to be intrinsic membrane proteins, at least three of which appeared to be phosphorylated by both these protein kinases. These phosphoproteins may represent membrane targets for multiple hormone or transmitter actions in skeletal muscle cells. Our results, therefore, suggest that protein phosphorylation systems, particularly those regulated by cAMP or calcium/phospholipid, may be more important in the regulation of sarcolemma function than hitherto believed.

Phosphorylation of multiple sites in a 15,000 dalton proteolipid from rat skeletal muscle sarcolemma, catalyzed by adenosine 3',5'-monophosphate-dependent and calcium/phospholipid-dependent protein kinases

This study reports a partial characterization of a 15,000 dalton (15 kDa) proteolipid present in rat skeletal muscle sarcolemma. The proteolipid is phosphorylated by both cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases, displays an isoelectric point (pI) of 5.9, and can be extracted from sarcolemma by acidified chloroform/methanol (2:1) or non-ionic detergents. Phosphoamino acid analysis and tryptic fingerprinting of the phosphorylated proteolipid indicate that both cyclic AMP- and calcium/phospholipid-dependent protein kinases predominantly phosphorylate serine residue(s) on a single tryptic peptide. Additivity experiments and thermolytic fingerprinting demonstrate a minimum of two distinct phosphorylation sites on the proteolipid, the phosphorylation of which is independently catalyzed by cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases in vitro. This sarcolemma proteolipid, which appears to be identified to a sarcolemma protein previously reported to be phosphorylated upon addition of insulin in a GTP-dependent manner (Walaas, O., Walaas, E., Rye-Alertsen, A. and Horn, R.S. (1979) Mol. Cell. Endocrinol. 16, 45-55), therefore represents a possible membrane target for those neuronal and hormonal stimuli which can regulate cyclic AMP-dependent or calcium/phospholipid-dependent protein kinase activities in skeletal muscle.

Alpha 1-adrenoceptor-mediated phosphoinositide breakdown and inotropic response in rat left ventricular papillary muscles

alpha 1-Adrenoceptor stimulation of rat left ventricular papillary muscles by phenylephrine in the presence of propranolol resulted in rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2) and a triphasic inotropic response in a concentration-dependent manner. The release of inositol trisphosphate (IP3) was maximum within 30 seconds and remained high for at least 30 minutes. The IP3 formation was associated with a rapid, but small, increase in contractile force followed by a transient decline in the contractility prior to the development of a sustained and more pronounced positive inotropic response. Inhibition of PI-4,5-P2 hydrolysis by the alpha 1-adrenergic antagonist prazosin or the PI-4,5-P2 phosphodiesterase inhibitor neomycin blocked all components of the inotropic responses. Combined addition of 2,3-diphosphoglyceric acid, a competitive inhibitor of IP3 phosphatase, with phenylephrine doubled the IP3 formation and potentiated the initial phases of inotropic responses but had no effect on the sustained positive inotropic response. Nifedipine and Mn2+ did not block the transient inotropic responses but inhibited the sustained positive inotropic response. alpha 1-Adrenoceptor stimulation resulted in restoration of slow responses in the high K+-depolarized muscles in the time course similar to that of the development in the sustained positive inotropic response. Addition of phorbol-12,13-dibutyrate alone or in combination with caffeine or A23187 failed to produce a sustained positive inotropic effect, but pretreatment with this phorbol ester (1-100 nM) for 30 minutes resulted in dose-dependent potentiation of alpha 1-adrenoceptor-mediated sustained positive inotropic effect associated with enhanced slow responses. These results suggest that the inotropic effects mediated by cardiac alpha 1-adrenoceptor stimulation occur through the phosphodiesteratic cleavage of PI-4,5-P2, such that IP3 may produce transient inotropic effects by mobilizing intracellular Ca2+, while diacylglycerol, along with cofactors that are also generated on alpha 1-adrenoceptor stimulation, may provoke a sustained positive inotropic effect by potentiating slow Ca2+ channels through activation of protein kinase C.

Stimulation of phosphatidylinositol metabolism in the isolated, perfused rat heart

Receptor-stimulated phosphatidylinositol turnover has been studied in isolated, perfused, [3H]inositol-labelled rat hearts by measuring accumulation of inositol phosphates in the presence of lithium chloride. Inositol phosphate accumulation was stimulated by norepinephrine (3 X 10(-5) M) and carbachol (10(-3) M), the increases averaging from 931 +/- 59 (n = 6, mean +/- SEM, cpm/g heart) to 4,165 +/- 609 (n = 6, p less than 0.01) for norepinephrine and to 1,853 +/- 354 (n = 6, p less than 0.05) for carbachol. The norepinephrine stimulation was antagonized by prazosin (10(-7) M) but not by propranolol (10(-7) M), indicating mediation via alpha 1-adrenoceptors. The carbachol stimulation was antagonized by atropine (10(-7) M). The stimulation by norepinephrine was significantly higher in right atria (837 +/- 151 to 6,614 +/- 1,210, n = 6, cpm/g tissue) than in other regions of the heart. Both norepinephrine and carbachol stimulated the formation of inositol monophosphate, inositol bisphosphate, and inositol trisphosphate with norepinephrine stimulation being detected as early as 15 seconds. Furthermore, the inositol trisphosphate was identified as the -1,4,5 isomer by anion exchange high-performance liquid chromatography. These data are consistent with the hydrolysis of phosphatidylinositol-(4,5)-bisphosphate yielding inositol-(1,4,5)-trisphosphate. Inositol-(1,3,4)-trisphosphate was not detected in heart preparations, suggesting unusual metabolism of inositol-(1,4,5)-trisphosphate in heart tissue.

Evidence for a lack of inositol--(1,4,5)trisphosphate kinase activity in norepinephrine-perfused rat hearts

The products of the phosphatidylinositol turnover pathway in norepinephrine-perfused hearts have been examined using high performance liquid chromatography. Inositol-1 phosphate, inositol-(1,4)bisphosphate and inositol-(1,4,5)-trisphosphate were all increased in response to norepinephrine stimulation. However, at perfusion times from 5 sec to 20 min there was no appearance of inositol-(1,3,4,5)tetrakisphosphate or inositol-(1,3,4)trisphosphate. This suggests the absence of the inositol-(1,4,5)trisphosphate phosphorylation/dephosphorylation pathway in heart and demonstrates that this secondary pathway is not essential to the functioning of the phosphatidylinositol turnover cycle.

Pertussis toxin-insensitive phosphoinositide hydrolysis, membrane depolarization, and positive inotropic effect of carbachol in chick atria

Muscarinic agonists can stimulate rather than inhibit cardiac muscle in some preparations. In left atria from hatched chicks, treatment with pertussis toxin reversed the membrane action of carbachol from hyperpolarization to depolarization and reversed the inotropic effect of carbachol from negative to positive. Acetylcholine also depolarized the membrane and increased the force of contraction in atria from pertussis-toxin-treated chicks although oxotremorine did not. These cholinergic responses were blocked by atropine but not by adrenoceptor antagonists, suggesting that they are mediated via muscarinic receptors and are not due to actions of endogenously released catecholamines. Muscarinic receptor stimulation leads to two distinct biochemical responses in chick atria: inhibition of adenylate cyclase and activation of phosphoinositide (PI) hydrolysis. The former is lost in atria from pertussis-toxin-treated chicks, whereas the PI response persists. The pharmacologic characteristics of the PI response resemble those of the depolarization and positive inotropic response. Both are insensitive to blockade by pertussis toxin, require high concentrations of carbachol, and are elicited by acetylcholine but not by oxotremorine. The present study suggests that muscarinic agonist-induced PI turnover may be responsible for the membrane depolarization and positive inotropic effects of carbachol and acetylcholine; that an increase in Na+ conductance underlies these responses; and that it is stimulated either by an increase of intracellular calcium mobilized by inositol triphosphate and/or by activation by protein kinase C.

Influence of BAY K-8644 on positive inotropic agents in guinea pig atrial muscle

This study examined the influence of BAY K-8644, a dihydropyridine Ca2+ agonist, on the positive inotropic effects of strophanthidin, isoproterenol, methoxamine and extracellular Ca2+ (Ca2+0) in atrial muscle isolated from guinea pig heart. BAY K-8644 enhanced both the maximum developed tension observed in the presence of strophanthidin and the sensitivity to its toxic effects. The maximum contractile force observed in the presence of methoxamine was also elevated by BAY K-8644 pretreatment; however, the ED50 value for methoxamine was not affected. The maximum contractile force elicited by BAY K-8644 alone or by strophanthidin or methoxamine in combination with BAY K-8644 was approximately the same as that produced by isoproterenol alone. The Ca2+ agonist did not alter the maximum developed tension elicited by increasing concentrations of isoproterenol or Ca2+0; however, it reduce both the ED50 for Ca2+0 and the concentration of isoproterenol necessary to produce maximum contractility. These results suggest that combinations of BAY K-8644 and cardiac glycosides can elevate contractile force to a level greater than that produced by cardiac glycosides alone.

A 16 kDa protein substrate for protein kinase C and its phosphorylation upon stimulation of vasopressin receptors in rat aortic myocytes

Phosphorylation induced by protein kinase C was examined in a plasma membrane fraction from rat aortic myocytes. Labelled phosphate incorporation produced by addition of kinase C to the membrane preparation allowed to identify a 16 kDa protein as the major substrate of the enzyme. This protein electrophoretically migrated with a protein phosphorylated by cAMP dependent protein kinase, but the two kinases produced phosphorylation of different sites since their effects were additive. Pretreatment of the myocytes with a kinase C activating phorbol ester or with vasopressin decreased further phosphate incorporation into the 16 kDa protein under the influence of exogenous kinase C. The results provide evidence that vasopressin produced in situ phosphorylation of the 16 kDa protein in rat aortic myocytes, with a time course and at concentrations consistent with a role of kinase C activation in the response of aortic myocytes to stimulation of V1 receptors.

Study of receptor-stimulated phosphatidylinositol hydrolysis in intact, perfused rat hearts

1. Phosphatidylinositol turnover has been measured in intact, perfused rat hearts by measuring generation of inositol phosphates following [3H]-inositol labelling. Stimulation of accumulations of inositol monophosphate, inositol bisphosphate and inositol trisphosphate were observed during perfusion with either noradrenaline (3 X 10(-5) mol/l) or carbachol (10(-3) mol/l). 2. Stimulation by noradrenaline was antagonized by prazosin (10(-7) mol/l) but not by propranolol (10(-7) mol/l), indicating mediation via alpha 1-adrenoceptors. Stimulation by carbachol was antagonized by atropine (10(-7) mol/l). 3. Transmural electrical stimulation of the hearts failed to increase inositol phosphate accumulation through alpha 1-adrenoceptors. A small stimulation mediated by muscarinic receptors was observed. Therefore alpha 1-adrenoceptors which stimulate phosphatidylinositol turnover probably do not have a synaptic localization in heart. 4. The development of methods for the study of phosphatidylinositol turnover in intact hearts will facilitate an investigation of relationships between this signal transduction pathway and cardiac function.

Relationship between beta-adrenoceptors and calcium channels in human ventricular myocardium

The stoichiometric relationship between adrenoceptors and saturable binding sites for 1,4-dihydropyridines in calcium channels was investigated in human ventricular myocardium. Membrane particles were prepared from heart specimens of patients undergoing open heart surgery. The patients suffered from hypertrophic obstructive cardiomyopathy (HOCM) or mitral valve disease. Using [3H]-prazosin and [125I]-2-beta-hydroxy-3-iodiphenyl-ethyl-aminoethyl tetralone ([125I]-HEAT) as labels we detected only a marginal density of alpha 1-adrenoceptors, regardless of disease. No alpha 2-adrenoceptors were detected with [3H]-rauwolscine. In HOCM patients we estimated 72 +/- 10 fmol mg-1 (n = 12) beta-adrenoceptors labelled with [3H]-(-)-dihydroalprenolol and 74 +/- 5 fmol mg-1 (n = 2) beta-adrenoceptors labelled with [125I]-(-)-iodocyanopindolol; the equilibrium dissociation constants KD, were 1.2 +/- 0.2 nmol l-1 for [3H]-(-)-dihydroalprenolol and 7 +/- 1 pmol l-1 for [125I]-(-)-iodocyanopindolol. In patients with mitral valve disease we estimated 84 +/- 11 fmol mg-1 (n = 3) labelled with [3H]-(-)-dihydroalprenolol and 66 +/- 13 fmol mg-1 (n = 2) labelled with [125I]-(-)-iodocyanopindolol. The KD values were 1.8 +/- 0.6 nmol l-1 for [3H]-(-)-dihydroalprenolol and 8 +/- 2 pmol l-1 for [125I]-(-)-iodocyanopindolol. In 14 HOCM patients we estimated 107 +/- 12 fmol mg-1 calcium channel sites labelled with [3H]-nimodipine with a KD of 280 +/- 4 pmol l-1. In 5 patients with mitral valve disease the density of calcium channel sites labelled with [3H]-nimodipine was 78 +/- 5 fmol mg-1 with a KD of 290 +/- 20 pmol l-1, In HOCM patients the density of calcium channel sites labelled with the benzoxadiazol 1,4-dihydropyridine ([3H]-(+)-PN 200-110) was 1.6 fold of that labelled with [3H]-nimodipine with a KD of 84 +/- 11 pmol l-1. In a group of 4 HOCM patients in which calcium channels were labelled with [125I]-iodipine, the density of sites was 1.37 +/- 0.07 fold the density of sites labelled by [3H]-(+)-PN 200-11-. The KD value of [125I]-iodipine was 246 +/- 16 pmol-1. (+)-PN 200-110 was approximately 100 fold more potent than (-)-PN 200-110 as a competitor of [125I]-iodipine binding. For the HOCM group a significant correlation was found between beta-adrenoceptor density and calcium channel density, whereas in the mitral valve group no such correlation was found. This does not prove that there is causal interaction leading to a relationship between the density of beta-adrenoceptors and calcium channels. However, because positive inotropic effects of catecholamines mediated by beta-adrenoceptors are associated with opening of calcium channels, this suggests that the density of both beta-adrenoceptors and calcium channels could be co-regulated.