ACETYLCHOLINE INHIBITS CA2-CYCLASE IN FROG CARDIAC MYOCYTES( CURRENT BY ACTING EXCLUSIVELY AT A SITE PROXIMAL TO ADENYLYL)

1. The effects of acetylcholine (ACh) on the L-type Ca2+ current (I-Ca ) stimulated by isoprenaline (Iso) or forskolin (Fsk) were examined in frog ventricular myocytes using the whole-cell patch-clamp technique and a double capillary for extracellular microperfusion. 2. The exposu re of one half of the cell to 1 mu M Iso produced a half-maximal incre ase in I-Ca since a subsequent application of Iso to the other half in duced an additional effect of nearly the same amplitude. Similarly, ad dition of 1 mu M ACh to only one half of a cell exposed to Iso on both halves reduced the effect of Iso by only similar to 50%. 3. When 10 m u M Iso or 30 mu M Fsk were applied to a Ca2+-free solution on one hal f of the cell, I-Ca was increased in the remote part of the cell where adenylyl cyclase activity was not stimulated. However, addition of AC h (3-10 mu M) to the remote part had no effect on I-Ca, while addition of ACh to the part of the cell exposed to Iso or Fsk strongly antagon ized the stimulatory effects of these drugs. 4. Our data demonstrate t hat ACh regulates I-Ca by acting at a site proximal to adenylyl cyclas e in frog ventricular cells. We conclude that the muscarinic regulatio n of I-Ca does not involve any additional cAMP-independent mechanisms occurring downstream from cAMP generation.