Effects of lobeline on slow action potential and force of contraction of guinea pig papillary muscle

Intracellular microelectrode techniques were used to study the effects of N1-receptor agonist lobeline on the slow action potentials (SAP) and the force of contraction (FC), induced by histamine, BaCl2 and aminophylline, in catecholamine-depleted guinea pig papillary muscles in the presence of an M-cholinergic receptor antagonist atropine (3 mumol.L-1). In these preparations lobeline (1-64 mumol.L-1) suppressed, in a dose-dependent manner, the action potential amplitude (APA), the action potential duration (APD), the maximal upstroke velocity (Vmax) and FC. Elevation of calcium concentration outside the myocardiac cells to 3.6 mmol.L-1 antagonized the effects of lobeline in different degrees. These results suggest that the lobeline inhibits the slow inward current Isi of myocardiac cells.