Intracellular ATP slows time-dependent decline of muscarinic cation current in guinea pig ileal smooth muscle

The effects of intracellular nucleotide triphosphates on time-dependent changes in muscarinic receptor cation currents (I-cat) were investigated using the whole cell patch-clamp technique in guinea pig ileal muscle. In the absence of nucleotide phosphates in the patch pipette, I-cat evoked every 10 min decayed progressively. This decay was slowed dose dependently by inclusion of millimolar concentrations of ATP in the pipette. This required a comparable concentration of Mg2+, was mimicked by UTP and CTP, and was attenuated by simultaneous application of alkaline phosphatase or inhibitors of tyrosine kinase. In contrast, a sudden photolytic release of millimolar ATP (probably in the free form) caused a marked suppression of I-cat. Submillimolar concentrations of GTP dose dependently increased the amplitude of I-cat as long as ATP and Mg2+ were in the pipette, but, in their absence, GTP was ineffective at preventing I-cat decay. The decay of I-cat was paralleled by altered voltage-dependent gating, i.e., a positive shift in the activation curve and reduction in the maximal conductance. It is thus likely that ATP exerts two reciprocal actions on I-cat, through Mg2+-dependent and -independent mechanisms, and that the enhancing effect of GTP on I-cat is essentially different from that of ATP.