A dual effect of melatonin on smooth muscle: role of tyrosine protein kinase

Abstract

High affinity melatonin binding sites have been localized in quail caecum (Poon AMS, et al, J Pineal Res 1997;23/5). In this study, the muscular response of the same subject to melatonin was tested. Under normal conditions (Krebs, 37 ° C, pH 7.45) strips and rings from quail caecum developed spontaneous phasic contractions. Neither activator (phorbol ester) nor inhibitor (calphostin C) of protein kinase C (PKC) were able to modify these contractile responses. Melatonin (1 pM–100 nM) similar to epidermal growth factor (EGF) (50–200 ng/ml), which is known to activate receptor tyrosine kinase, potentiated both the amplitude and frequency of spontaneous contractions in strips and rings. Further increase of concentrations of both agonists evoked the opposite effect. Nifedipine (100 nM), or cromakalim (3 ÌM, opener of K-channels), or genistein (40 ÌM, inhibitor of tyrosine kinases) completely suppressed spontaneous contractile responses. In contrast, the blocker of K-channels iberiotoxin (25 nM) enhanced the frequency of contractions in rings. Subsequent application of melatonin and EGF reversed this iberiotoxin-induced influence. In rings precontracted with KCl (30 mM), 10 ÌM melatonin and 0.4 Ìg/ml EGF potentiated a muscular tone up to 0.64 + 0.08 g (n = 10) which can be inhibited either by genistein or by nifedipine. Conclusion: These results indicate that spontaneous phasic contractile responses in quail caecum are a function of the activity of L-type Ca-channels and K-channels which seem to be regulated by receptor tyrosine kinase rather than PKC. The binding of melatonin to its receptor is to modulate both the phasic contractions and K-induced response, probably via the activation of protein tyrosine kinase. It is suggested that the dual effect of melatonin on spontaneous contractions is due to the initiation of two competitive events (an increase of inward Ca-current and outward K-current through channels) by this tyrosine kinase.