Effects of isoprenaline on the contraction-relaxation cycle in the cat trachea

Effects of isoprenaline (Isop) on the contractile properties of the smooth muscle cells of cat trachea were investigated using intact and chemically skinned muscle preparations and an isometric tension recording method. In the intact muscle preparations, Isop 3 X 10(-10) or 3 X 10(-9) M significantly suppressed the amplitude of tonic contractions evoked by acetylcholine (ACh) 10(-7) M or 10(-5) M, respectively. Following treatment of the tissue with Ca2+-free 2 mM EGTA-containing solution after depletion of stored Ca2+ with caffeine, 2.5 mM Ca2+ was applied for 5 min (procedure 1), and subsequently 10 mM caffeine was applied in Ca2+-free 2 mM EGTA containing solution. The object was to estimate the amount of stored Ca2+ during procedure 1 from the amplitude of the caffeine (10 mM)-induced contraction (procedure 2). Isop, applied during procedure 1, did not affect the amplitude of the caffeine-induced contraction; however, when applied during procedure 2, this agent (10(-8)M) significantly suppressed the amplitude of the caffeine-induced contraction to about 90% of the control value. ACh (10(-5)M), applied during procedure 1, evoked phasic and tonic contractions. Isop (10(-8)M), applied simultaneously with ACh (10(-5)M), suppressed the amplitude of the ACh-induced contraction yet increased the amplitude of contraction evoked by the subsequent application of caffeine 10 mM (procedure 2). Effects of conditioning application of ACh (10(-7) or 10(-5)M) on the caffeine-induced contraction were observed in the presence or absence of Isop during procedure 2. When ACh 10(-5)M was used, subsequent application of caffeine 10 mM evoked no mechanical response, in control conditions. However, after the pretreatment of the tissue with Isop during procedure 2, the amplitude of the ACh (10(-5)M)-induced contraction was not affected, yet the subsequent application of caffeine (10 mM) evoked minute but discrete contractions, indicating that Isop did enhance the sequestration of free Ca2+ into the storage sites. In the saponin-treated skinned muscles, the minimum concentration of Ca2+ required to produce contraction was 1 X 10(-7)M, and the maximum contraction was obtained with 1 X 10(-5)M Ca2+. Isop (10(-6)M) had no effect on the relationship between free-Ca2+ and the amplitude of the contraction. However, simultaneous application of high concentrations of cyclic AMP (10(-4)M) and cyclic AMP-dependent protein kinase (50 micrograms ml-1) significantly suppressed contractions evoked by 3 X 10(-7) or 10(-5)M Ca2+. 8 These results indicate that Isop suppresses the contraction evoked by various agonists in the cat trachea, mainly through sequestration of Ca2+ into the intracellular storage sites, rather than by direct or indirect (through cyclic AMP) actions on the contractile proteins.