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

1. It has been suggested that pituitary adenylate cyclase activating peptid e (PACAP) may be involved in the non-adrenergic, non-cholinergic (NANC) inh ibitory response of longitudinal muscle of rat distal colon. In this study, we have investigated the intracellular mechanism of PACAP-induced relaxati on in this muscle. 2. PACAP induced an apamin-sensitive relaxation of the longitudinal muscle. The tyrosine kinase inhibitors genistein at 10 mu M and tyrphostin 25 at 3 0 mu M, but not the cyclic AMP-dependent protein kinase inhibitor R-p-8-bro moadenosine-3',5'-cyclic monophosphorothioate at 30 mu M significantly inhi bited the PACAP-induced relaxation to 60% and 25% of control values, respec tively. PACAP did not increase the cyclic AMP content of the muscle. 3. Tyrphostin 25 at 10 mu M significantly inhibited the relaxation of longi tudinal muscle induced by electrical field stimulation (EFS), to 50% of con trol values. Apamin at 1 mu M, an antagonist of small conductance Ca2+-acti vated 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 me mbrane of the muscle. Tyrphostin 25 at 3 mu M inhibited this PACAP-induced hyperpolarization. Tyrphostin 25 at 10 mu M and genistein at 10 mu M inhibi ted the apamin-sensitive inhibitory junction potentials induced by a single pulse of EPS. 5. The PACAP-induced relaxation of longitudinal muscle occurred with a conc omitant decrease in intracellular Ca2+ levels ([Ca2+](i)). Tyrphostin 25 at 10 mu M and apamin at 1 mu M abolished these PACAP-induced responses. 6. From these findings it is suggested that the activation of tyrosine kina se is involved in PACAP-induced relaxation of longitudinal muscle from rat distal colon, 'upstream of' the activation of apamin-sensitive K+ channels.