Effects of nicotinamide on cardiac contraction force and slow inward current

The effects of nicotinamide (Nic) on cardiac contraction force and on slow inward current (Isi) in guinea pigs were studied with atrial strips and voltage clamp techniques. Verapamil (Ver), MnCl2, nifedipine (Nif), and propranolol (Pro) depressed markedly the positive inotropic effect induced by Nic in a noncompetitive manner. The pD2 values of Ver, MnCl2, Nif, and Pro were 6.19, 3.41, 5.00, and 6.43, respectively. The action of Nic was reduced by a low Ca2+ Tyrode solution and disappeared in a Ca(2+)-free solution. Nic 33 mmol.L-1 elevated the Isi from 6.5 +/- 1.3 microA to 10.3 +/- 2.2 microA. The results suggest that Nic promotes the Ca2+ influx and its site of action is different from that of both Pro and the calcium antagonists.