EFFECTS OF HIGH-ENERGY PHOSPHATES ON CARBACHOL-EVOKED CATIONIC CURRENT IN SINGLE SMOOTH-MUSCLE CELLS FROM GUINEA-PIG ILEUM

1. Single smooth muscle cells from the longitudinal muscle layer of guinea-pig small intestine were voltage clamped in the whole-cell recording mode with patch pipettes. The cationic current (I-cat) evoked by application of 50 mu M carbachol (CCh) was examined when free internal calcium in the cell was 'clamped' at 10(-7) M with 20 mM BAPTA. The effects of varying the composition of the pipette solution were studied. 2. Phosphocreatine (PCr, 6 mM) added to the pipette solution increased I-cat by about 7-fold (to near 620 pA); lower concentrations had similar, generally lesser, effects. Na(2)ATP (3 or 6 mM) with or without 5 mM MgCl2 was much less effective than phosphocreatine alone. Addition of 3 mM Na(2)ATP reduced I-cat, whether or not phosphocreatine was present. 3. Creatine (6 mM) with or without 2 mar Na(2)ATP was less effective than phosphocreatine in maintaining I-cat. 4. GTP (0.1 mM) did not affect I-cat evoked by CCh, whether phosphocreatine was present or not. 5. GTP gamma S (0.2 mM) included in pipette solution mimicked the effect of CCh and evoked I-cat independently of whether PCr was present or not in the pipette solution. Including 5 mM ATP in the pipette reduced this current, whereas 5'-adenylyl imidodiphosphate (AMP-PNP) and ADP were without effect. 6. The results show that phosphocreatine increases membrane channel responsiveness to receptor activation and that ATP above 2 mM suppresses it.