Abstract

The effects of cromakalim, an ATP-sensitive K+ channel activator, on changes in cytosolic calcium concentration [( Ca++]i) and tension induced by acetylcholine (ACh; 0.1-10 microM) were examined in swine tracheal smooth muscle. Cromakalim (10 microM) hyperpolarized muscle cells by approximately 18 mV from -58 mV (resting membrane potential) to -76 mV. Cromakalim relaxed muscle contractions evoked by ACh at a concentration of 0.1 microM, but not at higher concentrations. Measurement of [Ca++]i using Fura-2 demonstrated that except at 0.1 microM ACh, cromakalim did not alter peak increases in [Ca++]i. At 0.1 microM ACh, the peak transient was decreased, but not eliminated. Cromakalim reduced steady-state increases in [Ca++]i at ACh less than or equal to 1 microM, but not 10 microM ACh. Tension was similarly affected. These data suggest that ACh-induced increases in steady-state [Ca++]i and tension are inhibited by cromakalim-induced hyperpolarization. The initial ACh-induced transient increase in [Ca++]i is not greatly altered. Cromakalim did not alter the transient peak tension and [Ca++]i relationship. The relationship between steady-state [Ca++]i/tension (EC50 = 321 nM) obtained for control, cromakalim inhibition and after glibenclamide reversal of cromakalim inhibition falls to the left of the peak transient [Ca++]i/tension relationship (EC50 = 587 nM). Thus, the Ca++ sensitivity of the contractile proteins during steady-state stimulation by ACh was increased from that at rest. We conclude that electromechanical coupling is important in ACh-induced contraction at concentrations less than 1 microM. Pharmacomechanical coupling with little or no sensitivity to changes in potential is important at higher ACh concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)