Slow Ca2+ channels neither contribute to upstroke of action potential nor to pacemaker potential in spontaneously active freshly isolated three day embryonic chick ventricle.

Contribution of slow Ca2+ channels to the upstroke of action potential (AP) and pacemaker potential was studied by observing the effects of Ca2+ channel activators- high [Ca2+]0, Bay-K-8644, isoproterenol, forskolin and dibutyryl-cAMP on spontaneous AP of freshly isolated 3 day embryonic chick ventricle (3 day ECV). The spontaneous APs showed maximal upstroke velocity (+Vmax), maximum diastolic potential (MDP), overshoot (Eov) and AP duration at -20 mv (APD20) of 42.60 +/- 2.40 V/sec, -59.05 +/- 0.95 my, 16.30 +/- 0.53 mv and 70.32 +/- 4.60 msec, respectively (an average value of 35 preparations). Bay-K-8644 (0.1-0.8 microM), isoproterenol (5-10 pM) and forskolin (0.1-2.0 microM) induced a concentration-dependent increase in APD20 and Eov without affecting +Vmax. Dibutyryl-cAMP (1 microM) also enhanced the APD20 and Eov and had no effect on +Vmax. Elevation of [Ca2+]0 from 0.6 mM to 9.6 mM caused a concentration-dependent increase in APD20 and Eov leaving +Vmax unaltered. Elevated [Ca2+] and the other Ca2+ channel activators had no significant effect on MDP in above concentration range. Increase in APD20 and Eov could be explained at least by activation of slow Ca2+ channels but the lack of any change in +Vmax clearly suggests that the slow Ca2+ channels do not contribute to the upstroke of AP. All these interventions reduced the rate of spontaneous firing without any noticeable effect on MDP. This finding shows that the slow Ca2+ channels also do not contribute directly to the generation of pacemaker potential in spontaneously active freshly isolated 3 day ECV.

Verapamil and TTX inhibit +Vmax but differentially alter the duration of action potential of adult chicken ventricular myocardium.

Verapamil, a Ca2+ channel blocker, is also reported to block Na+ channels in mammalian heart and to modulate the repolarisation phase of cardiac action potential (AP). The Na+ channel blocking activity of verapamil and its implication to changes in repolarisation were studied on chicken ventricular strips where upstroke is due to highly TTX sensitive Na+ channels. At low doses verapamil (0.5-5 micro M) and TTX (0.1-0.5 nM) did not cause any significant effect on resting membrane potential (Em), maximal upstroke velocity (+Vmax) or AP duration (ADP). Higher concentrations of both verapamil (10-320 micro M) and TTX (1-40 nM) caused dose-dependent decrease in +Vmax and overshoot (Eov) without any change in Em. EC50 for the inhibitory effect of verapamil and TTX on +Vmax was 140 microM and 14 nM respectively. Na+ channels in adult chicken ventricular myocardium, therefore, seem to be more sensitive to TTX than their mammalian counterpart. Higher dosage of verapamil are needed to block Na+ channels in adult avian heart as reported for mammalian myocardium. Both verapamil and TTX caused dose-dependent changes in APD at-20 mV (ADP20) and at 90% repolarisation (APD90). TTX (1-40 nM) produced a decrease of 5-13% in APD20 and 4-12% in APD90 indicating a uniform hastening of the repolarisation process. Verapamil (10-320 micro M), however, induced 6-38% decrease in APD20 but 5-12% increase in APD90. Regression analysis of the relationship between changes in +Vmax and APD20 and APD90 in presence of TTX and verapamil exhibit significant linear correlation r for APD20 and APD90, being +0.965 for TTX and +0.978 and-0.898 for verapamil respectively. A linear correlation between inhibition of +Vmax and reduction in APD for TTX indicates the possibility that Na+ channel linked mechanism(s) underlie repolarisation process. Verapamil induced decrease in APD20 and increase in APD90 could be explained by the block of Na+/Ca2+ and K+ channels respectively.