Positive chronotropic responses of rabbit sino-atrial node cells to flash photolysis of caged isoproterenol and cyclic AMP

The kinetics of onset and the intracellular biochemical signalling mechanisms which are responsible for the positive chronotropic effect of sympathetic stimulation in rabbit cardiac pacemaker cells were examined by using flash photolysis of caged isoproterenol (ISO) and cyclic AMP (cAMP). When caged ISO (10 $\mu $M) was present in the superfusate, a single ultraviolet flash caused gradual increases in the spontaneous beating frequency and action potential height of S-A node cells. Both these effects developed after an initial latency of approximately 5 s. Photorelease of ISO also increased the L-type Ca$^{2+}$ current (I$_{\text{Ca-L}}$) with a time-course similar to that of the changes in action potential waveform and heart rate. All of these ISO-induced effects were blocked completely by 1 $\mu $M propranolol, demonstrating that they were $\beta $-adrenergic responses. Flash photolysis of caged cAMP (50 $\mu $M) also resulted in increased firing frequency and I$_{\text{Ca-L}}$. However, these responses to cAMP developed with little or no latency. Intracellular dialysis with a selective inhibitor of the cAMP-dependent protein kinase, Rp-cAMPS, completely abolished the increase in I$_{\text{Ca-L}}$ demonstrating that it is mediated exclusively via cAMP-dependent activation of protein kinase A, as opposed to a direct G-protein mediated mechanism.