Effects of nicotinamide on action potentials of sinoatrial node cell and depolarized papillary muscle cell of guinea pig

The effects of nicotinamide (Nic) on action potentials (AP) of guinea pig SA node cell, normal papillary muscle cell and papillary muscle cell depolarized by high K+ were studied. Nic 33 mmol/L increased the slope of phase 4 of AP of SA node cell by 43%; APD90 of normal papillary muscle cell was shortened by 10%; Vmaxs of phase 0 of AP of depolarized papillary muscle cell was not much changed; Vmaxf of phase 0 of AP of depolarized papillary muscle cell was decreased by 21%; excitability of the depolarized papillary muscle was inhibited. Verapamil 0.22 micromol/L decreased the slope of phase 4 and Vmax of AP of SA node cell respectively by 44% and 80%; Vmaxs of AP of the depolarized papillary muscle cell was decreased by 23%; Vmaxf was not significantly changed.
The results suggest: 1) There may be a difference between Isi in the phase 4 of SA node AP and Isi in the phase 0 of SA mode AP and Vmaxs of AP of the partially depolarized papillary muscle cell; 2) The inhibition of Nic on excitability of myocardium may be related to the effect on Na+ channel.