Intracellular resistance in rat papillary muscle: interaction between cyclic AMP and calcium

The influence of isoproterenol (10(-9)M) and high calcium solution (6 mM) on the intracellular longitudinal resistance (ri) on rat papillary muscle was investigated. The muscles were stimulated at 1 Hz. Isoproterenol (10(-5)M) reduced ri within 10 s while high calcium solution (6 mM) increased ri appreciably. In muscles previously exposed to high calcium solution, isoproterenol increased ri further. This increment of ri, which was suppressed by verapamil (10(-5) M), indicates that when the inward movement of calcium through surface cell membrane is appreciably enhanced, the increase in free (Ca)i counteracts the effect of cyclic AMP on ri. Forskolin (10(-5)M), an activator of adenyl cyclase, also reduced ri in muscles immersed in normal saline solution. The results indicate that cyclic AMP and calcium have opposite effects on the control of ri.