Modulation of spontaneous electrical activity of freshly isolated 3-day embryonic chick ventricle by cAMP and cGMP

Effects of cyclic nucleotides 8-Bromo-cAMP and 8-Bromo-cGMP (membrane permeable analogs of cAMP and cGMP) were examined on action potential (AP) configuration and rate of spontaneous firing of the freshly isolated 3-day embryonic chick ventricle (ECV) to assess the role of L-type slow Ca2+ channels in upstroke of AP and spontaneous electrical activity (pacemaker potential). The 3-day ECV exhibited prominent automaticity and spontaneous APs characterized by maximum upstroke velocity (+V(max)), maximum diastolic potential (MDP), overshoot (E(ov)), AP duration at -20 mV (APD20) and cycle length (CL) of 33.09 +/- 3.18 V/sec, -63.77 +/- 1.17 mV, 17.40 +/- 0.91 mV, 51.20 +/- 3.05 m sec and 795 +/- 150 m sec, respectively (n = 10 preparations). 8-Br-cAMP (1 mM) caused significant increase in E(ov) and APD20 (37% and 56%, respectively, p < 0.01), but failed to produce any stimulatory effect on +V(max) and MDP. Surprisingly, 8-Br-cAMP produced negative chronotropic effect on spontaneous firing (automaticity) and enhanced the CL significantly by 43% (p < 0.05). 8-Br-cGMP, however, had no effect on AP configuration and the rate of spontaneous firing. The present findings with 8-Br-cAMP suggest that L-type slow Ca2+ channels do not contribute to upstroke of AP and pacemaker potential of spontaneously firing freshly isolated 3-day ECV. The negative chronotropic effect of 8-Br-cAMP suggests that the ionic mechanism underlying pacemaker potential is [Ca]i-dependent. However, the lack of any effect of 8-Br-cGMP on spontaneous electrical activity of freshly isolated 3-day ECV indicates that cGMP does not modulate the basal Ca2+ channel activity in young embryonic myocardium.