POSITIVE CHRONOTROPIC RESPONSES OF RABBIT SINOATRIAL NODE CELLS TO FLASH-PHOTOLYSIS OF CAGED ISOPROTERENOL AND CYCLIC-AMP

The kinetics of onset and the intracellular biochemical signalling mec hanisms which are responsible for the positive chronotropic effect of sympathetic stimulation in rabbit cardiac pacemaker cells were examine d by using flash photolysis of caged isoproterenol (ISO) and cyclic AM P (cAMP). When caged ISO (10 CIM) was present in the superfusate, a si ngle ultraviolet flash caused gradual increases in the spontaneous bea ting frequency and action potential height of S-A node cells. Both the se effects developed after an initial latency of approximately 5 s. Ph otorelease of ISO also increased the L-type Ca2+ current (I-Ca-L) with a time-course similar to that of the changes in action potential wave form and heart rate. All of these ISO-induced effects were blocked com pletely by 1 mu M propranolol, demonstrating that they were P-adrenerg ic responses. Flash photolysis of caged cAMP (50 mu M) also resulted i n increased firing frequency and I-Ca-L. However, these responses to c AMP developed with little or no latency. Intracellular dialysis with a selective inhibitor of the cAMP-dependent protein kinase, Rp-cAMPS, c ompletely abolished the increase in I-Ca-L demonstrating that it is me diated exclusively via cAMP-dependent activation of protein kinase A, as opposed to a direct G-protein mediated mechanism.