Tyrosine kinase involvement in apamin‐sensitive inhibitory responses of rat distal colon

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Abstract
1 It has been suggested that pituitary adenylate cyclase activating peptide (PACAP) may be involved in the non-adrenergic, non-cholinergic (NANC) inhibitory response of longitudinal muscle of rat distal colon. In this study, we have investigated the intracellular mechanism of PACAP-induced relaxation in this muscle. 2 PACAP induced an apamin-sensitive relaxation of the longitudinal muscle. The tyrosine kinase inhibitors genistein at 10 μm and tyrphostin 25 at 30 μm, but not the cyclic AMP-dependent protein kinase inhibitor Rp-8-bromoadenosine-3′,5′-cyclic monophosphorothioate at 30 μm significantly inhibited the PACAP-induced relaxation to 60% and 25% of control values, respectively. PACAP did not increase the cyclic AMP content of the muscle. 3 Tyrphostin 25 at 10 μm significantly inhibited the relaxation of longitudinal muscle induced by electrical field stimulation (EFS), to 50% of control values. Apamin at 1 μm, an antagonist of small conductance Ca2+-activated K+ channels, also inhibited the relaxation, to 42% of control values. The inhibitory effects of tyrphostin 25 and apamin were not additive (44% of control values). 4 PACAP induced an apamin-sensitive, slow hyperpolarization of the cell membrane of the muscle. Tyrphostin 25 at 3 μm inhibited this PACAP-induced hyperpolarization. Tyrphostin 25 at 10 μm and genistein at 10 μm inhibited the apamin-sensitive inhibitory junction potentials induced by a single pulse of EFS. 5 The PACAP-induced relaxation of longitudinal muscle occurred with a concomitant decrease in intracellular Ca2+ levels ([Ca2+]i). Tyrphostin 25 at 10 μm and apamin at 1 μm abolished these PACAP-induced responses. 6 From these findings it is suggested that the activation of tyrosine kinase is involved in PACAP-induced relaxation of longitudinal muscle from rat distal colon, ‘upstream of’ the activation of apamin-sensitive K+ channels.