FREQUENCY-MODULATION OF ACETYLCHOLINE-INDUCED OSCILLATIONS IN CA-ACTIVATED CL- CURRENT BY CAMP IN TRACHEAL SMOOTH-MUSCLE(+ AND CA++)

The effects of adenosine 3':5'-cyclic monophosphate (cAMP) on acetylch oline (ACh)-induced oscillations in intracellular calcium concentratio n ([Ca++](i)) and Ca++-activated Cl- current (Cl-Ca current) were dete rmined in isolated tracheal smooth muscle cells. Whole-cell current wa s measured in individual smooth muscle cells with patch clamp methodol ogy. At a holding potential of -80 mV, ACh (0.1 mu M) elicits base lin e-separated oscillations in Cl-Ca current which correlate with oscilla tions in [Ca++](i). The addition of the beta adrenoceptor agonist isop roterenol (ISO) (10 nM to 1 mu M) in the continued presence of ACh cau sed a concentration-dependent decrease in the frequency of the oscilla tions in Cl-Ca current with significant reductions in oscillation freq uency of 21.4 and 81.5% in the presence of 0.01 and 0.1 mu M ISO, resp ectively (P <.05). This effect was mimicked by both forskolin (FSK) (3 mu M) and 3-isobutyl-1-methylxanthine (IBMX) (30 mu M). ISO and forsk olin also inhibited ACh-induced oscillations in [Ca++](i) measured by confocal fluorescence microscopy in non-voltage-clamped cells loaded w ith the Ca++-sensitive dye, fluo3. The inhibition of ACh-induced oscil lations in Cl-Ca current by ISO was partially reversed by increasing e xtracellular Ca++. These data are consistent with previous observation s that the frequency of ACh-induced oscillations in [Ca++](i) and Cl-C a current is dependent on the concentration of extracellular Ca++ and the influx of Ca++ through a verapamil-sensitive pathway. Moreover, th ese results lend support to the hypothesis that beta adrenoceptors inh ibit the ACh-induced increase in [Ca++](i) through a cAMP-dependent me chanism that inhibits Ca++ influx and is independent of changes in mem brane potential.