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

Receptor crosstalk: effects of prolonged carbachol exposure on beta 1-adrenoceptors and adenylyl cyclase activity in neonatal rat ventricular myocytes

Supersensitivity of adenylyl cyclase after exposure to inhibitory agonists is a general means of cellular adaptation. We hypothesized that such "crosstalk" between muscarinic cholinergic agonists, beta 1-adrenoceptors, and adenylyl cyclase may be an important mechanism of cardiac adaptation to interventions that enhance vagal activity. We used primary cultures of neonatal rat ventricular myocytes and measured beta-adrenoceptors by radioligand binding and adenylyl cyclase activity by a single column method. Carbachol induced a time- and dose-dependent reversible decrease in cell surface beta 1-adrenoceptors. The peak effect occurred after 20 h of exposure to 100 microM carbachol which caused a decrease in the maximum number of binding sites for the beta-adrenoceptor antagonist 3H-CGP-12177 from 42.3 +/- 3.4 to 33.0 +/- 2.6 fmol/mg protein (n = 12, P < 0.03) without a change in antagonist affinity. Loss of cell surface receptors was prevented by atropine and by the protein kinase C inhibitor H7. The decrease in cell surface receptors was not accompanied by receptor internalization as assessed by equilibrium binding experiments in a cytosolic fraction using 125I-iodocyanopindolol. In contrast to the well-known acute inhibitory effects of carbachol on adenylyl cyclase activation, prolonged carbachol exposure preserved (-)-isoprenaline-stimulated adenylyl cyclase activity and enhanced postreceptor stimulated adenylyl cyclase activity. Carbachol did not further enhance adenylyl cyclase activity after pretreatment with pertussis toxin. The protein kinase C inhibitor chelerythrine prevented the carbachol induced enhancement of forskolin-stimulated adenylyl cyclase activity. We conclude that prolonged incubation with carbachol in rat neonatal ventricular myocytes causes a reduction in cell surface beta 1-Adrenoceptor density. beta 1-Adrenoceptor-mediated adenylyl cyclase activity is preserved and postreceptor-mediated adenylyl cyclase activity is augmented. Our data suggest that carbachol-stimulated protein kinase C activity may play a key role in the prolonged muscarinic regulation of adenylyl cyclase activity.

Effects of phorbol ester on contraction, intracellular pH and intracellular Ca2+ in isolated mammalian ventricular myocytes

1. We have investigated the actions of certain phorbol esters on the intracellular pH, intracellular Ca2+ and contractility of isolated rat and guinea-pig cardiac myocytes. Intracellular pH was measured using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and intracellular Ca2+ was measured using Fura-2. 2. Application of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (also called phorbol 12-myristate 13-acetate) (TPA) (which activates protein kinase C) to rat cardiac myocytes significantly increased cell shortening by 116 +/- 34% (n = 8) (p less than 0.02). The rate of change of cell length during contraction (i.e. +dL/dt) increased from 67.2 +/- 8.7 microns/s to 127.7 +/- 14.1 microns/s (n = 7). The rate of change of cell length during relaxation (-dL/dt) increased from 55.8 +/- 7.4 microns/s to 118.9 +/- 12.1 microns/s (n = 7). Time to peak shortening was unchanged. 3. Application of 4 alpha-phorbol 12,13-didecanoate, which does not activate protein kinase C, did not affect rat myocyte contractility. An insignificant decrease in contractility (by 7.5 +/- 7.5%) was observed (n = 5). The positive inotropic effect of TPA may therefore be evoked through an activation of protein kinase C. 4. In rat myocytes we have measured the changes of pHi and contractility (cell shortening) during an alkalosis and acidosis induced by exposure to and subsequent removal of NH4Cl both in the presence and absence of TPA. Recovery times from an acid load were significantly (p less than 0.05) enhanced by 15.1 +/- 6.9% (n = 13) in the presence of TPA. Recovery times of cell shortening were also more rapid (p less than 0.05) by an average of 59.1 +/- 10.6% (n = 5) in the presence of TPA. Recovery times were unchanged in the presence of 4-phorbol 12,13-didecanoate (which does not activate protein kinase C). 5. Since pHi recovery of an isolated myocyte from an acid load is partially inhibited by the presence of 1 mM-amiloride and inhibited by removing extracellular Na+ then it is suggested that, like pHi regulation in sheep heart Purkinje fibres, pHi recovery in rat cardiac ventricular myocytes is mainly through sarcolemmal Na(+)-H+ exchange. We suggest that in the presence of TPA the Na(+)-H+ exchange is stimulated. 6. The relationship between pHi and cell shortening is non-linear as has been observed by others in whole tissue preparations. The presence of TPA shifts the relationship upwards such that at any one pHi, cell shortening is greater.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional and autoradiographic evidence for endothelin 1 receptors on human and rat cardiac myocytes. Comparison with single smooth muscle cells

This study aimed to determine whether receptors for endothelin were present on the cardiac myocyte as well as on vascular smooth muscle cells. Low- and high-resolution autoradiography was performed using 125I-endothelin 1 on intact rat myocardium and samples of human ventricle obtained from explanted hearts at the time of transplant. In addition to specific binding to the smooth muscle of the blood vessel lumen, there was considerable binding associated with cardiac myocytes. To discover whether there was any functional correlate for this binding, muscle cells were isolated enzymatically from human and rat ventricle and from rat femoral artery, and their contractile characteristics were studied. Single cardiac cells were superfused with physiological saline at 32 degrees C, and their length change was displayed continuously on a chart recorder. Endothelin 1 had a pronounced effect on shortening in both rat and human myocytes. The contraction amplitude was approximately doubled in both cases, from 4.1 +/- 0.8% cell length to 8.1 +/- 1.3% for rat (mean +/- SEM, n = 9, p less than 0.001), and from 2.1 +/- 0.5% to 4.0 +/- 0.5% in human (n = 10, p less than 0.001). In rat, the magnitude of the effect was comparable to that of the alpha-adrenoceptor agonist phenylephrine. The maximum contraction amplitude of the human cells, produced by raising extracellular calcium to greater than 10 mM, was 11.4 +/- 1.1% cell length (n = 9), significantly greater than that produced by endothelin (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of lithium on the positive inotropic effect of phenylephrine and isoprenaline in guinea-pig heart

The influence of lithium on the positive inotropic effects mediated by alpha- and beta-adrenoceptor stimulation was studied in isolated guinea-pig ventricular preparations stimulated at 1 Hz. Lithium chloride (10(-3)-2 X 10(-2) M) shifted the concentration-response curve for the inotropic effect of phenylephrine to the right in a dose-dependent manner. [3H]Prazosin binding was not inhibited by 10(-2) M lithium. The antagonistic effect of lithium was almost completely prevented by the presence of 10(-2) M myoinositol, and was potentiated by 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol, an antagonist of myo-inositol. The positive inotropic effect of isoprenaline was completely unaffected by either 10(-2) M lithium, 10(-2) M myo-inositol, or 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol. Since it is known that lithium interferes with inositol phosphate metabolism, our results produce further indirect evidence of an involvement of inositol phosphates in the myocardial response to alpha-adrenoceptor, but not to beta-adrenoceptor, stimulation. Our findings also suggest that chronic lithium treatment could interfere with the adrenergic modulation of myocardial contractility in those physio-pathological conditions in which the role of myocardial alpha-adrenoceptors becomes predominant.