Synergism between cAMP and ATP in signal transduction in cardiac myocytes

Abstract

ATP transiently increases the intracellular Ca2+ concentration in cardiac myocyte suspensions. Pretreatment with norepinephrine (NE) greatly potentiates the ATP response. We performed experiments on adult rat myocyte suspensions loaded with fura-2 to investigate the mechanism of NE potentiation. We found that forskolin (an activator of adenylate cyclase), 3-isobutyl-1-methylxanthine (an inhibitor of phosphodiesterase), and permeative adenosine 3',5'-cyclic monophosphate (cAMP) analogues potentiate the increase in cytosolic Ca2+ concentration induced by ATP. NE, forskolin, and 8-(4-chlorophenylthio)-cAMP all increase Vmax of the Ca2+ response curve of ATP. Measurement of cAMP by radioimmunoassay confirmed that the changes in the ATP response were accompanied by an increase in cAMP. These results suggest that the noradrenergic potentiation of the ATP-induced Ca2+ mobilization involves cAMP as a second messenger. Patch-clamp studies of isolated myocytes showed that neither NE nor forskolin alters the inward current elicited by ATP, but rather they increase the duration of secondary slow action potentials elicited by ATP. NE also increases the Ca2+ current through L-type Ca2+ channels in the myocytes. We conclude that NE potentiates the ATP-induced Ca2+ transient by increasing cAMP levels and that one of the early events is the increase of the inward Ca2+ current during the action potential.

Involvement of Na+/Ca2+ exchanger in catecholamine-induced increase in intracellular calcium in cardiomyocytes

“…The isoproterenol-mediated increase in [Ca 2ϩ ] i , unlike that due to ATP, was markedly inhibited by propranolol, indicating the involvement of ␤-ARs in the isoproterenol-mediated increase in [Ca 2ϩ ] i . Similarly, Zheng et al (36) have shown that the isoproterenol-induced increase in the ratio of changes in fluorescence level (⌬F ATP /⌬F KCl ) was significantly attenuated by propranolol after the addition of ATP and KCl. The effect of ␤-AR blockade on isoproterenol-induced Ca 2ϩ -mobilization may be of some specific nature because basal [Ca 2ϩ ] i remained unaltered in the presence of propranolol.…”

“…The ATP-induced increase in [Ca 2ϩ ] i was also attenuated by treatment with these antagonists. Thus an increase in the amplitude of inward Ca 2ϩ current through the SL L-type Ca 2ϩ channels (36) and ␤-AR-cAMP-PKA pathway-mediated phosphorylation of L-type Ca 2ϩ channels (20) may be the mechanism of isoproterenol-and ATPmediated increase in Ca 2ϩ -influx. Although ␤-AR stimulationmediated Ca 2ϩ entry by L-type Ca 2ϩ channels has been linked with Ca 2ϩ release from the SR Ca 2ϩ stores (30), the isoproterenol-mediated potentiation of the ATP-induced increase in [Ca 2ϩ ] i seems to be independent of Ca 2ϩ release from the intracellular stores because ryanodine, an inhibitor of the SR Ca 2ϩ release channel (4,25), did not affect the isoproterenolmediated increase in [Ca 2ϩ ] i .…”

“…Although under physiological conditions Na ϩ /Ca 2ϩ exchanger is an important mechanism of Ca 2ϩ efflux (17), an increase in intracellular Na ϩ or a change in transsarcolemmal potential causes the reversal of the exchanger to mediate Ca 2ϩ entry (3). In this context, ATP has been shown to cause depolarization of the membrane (8,36) and the induction of 22 Na ϩ influx in the isolated cardiomyocytes (15). In addition, PKA-mediated phosphorylation of Na ϩ /Ca 2ϩ exchanger subsequent to increase in cAMP concentration by isoproterenol treatment (22) may be the mechanism of catecholamine-induced increase in [Ca 2ϩ ] i .…”

“…This increase of the ATP response by norepinephrine treatment has been demonstrated to be due to Ca 2ϩ entry through the sarcolemmal (SL) L-type Ca 2ϩ channels. In this regard, it is pointed out that norepinephrine and other catecholamines, including isoproterenol, have been shown to phosphorylate the L-type Ca 2ϩ channels and enhance the inward Ca 2ϩ current as a consequence of increased production of cAMP and the activation of protein kinase A (PKA) (36,37). The increased Ca 2ϩ entry through the SL membrane further triggers the release of Ca 2ϩ from the sarcoplasmic reticulum (SR) stores and thus results in the catecholamine-mediated increase in [Ca 2ϩ ] i in electrically stimulated cardiomyocytes (16).…”

Coupling of dual acid extrusion in the guinea‐pig isolated ventricular myocyte to alpha 1‐ and beta‐adrenoceptors.

“…This suggests that ATPo and catecholamines share common postreceptor transduction pathways. Given the fact that in many cell types ATPo and catecholamines can potentiate their individual effects (De Young & Scarpa, 1987;Zheng, Christie, De Young, Levy & Scarpa, 1992), and given that they are coreleased from noradrenergic terminals in the heart (Burnstock, 1981;Burnstock & Kennedy, 1986;see also Von Kiigelgen & Starke, 1991), the possibility must remain that much lower doses of both agents than hitherto tested, when combined, may influence acid extrusion from the cardiac cell. This raises the strong possibility that catecholamines may be important modulators of pHi regulation in the cardiac cell, not only under pathological conditions such as myocardial ischaemia and hypoxia (when local concentrations of catecholamines and ATPo may be extremely high; see Gordon, 1986 for review) but also under more normal physiological conditions.…”