An anti-adrenergic effect of melatonin on quail caecal smooth muscle: relationship between protein kinase A and tyrosine kinase

Abstract

Binging of 2[125I]-iodomelatonin, a specific melatonin agonist, to the quail caecum was studied. In addition, the effect of melatonin on the spontaneous contractile response in quail caecum and the possible mechanism of melatonin action involved were explored. A single population of specific and high affinity binding sites for melatonin (Kd = 24.6 B 1.1 pmol/l, n = 4; Bmax = 2.0 B 0.1 fmol/mg, n = 7) has been revealed in the crude caecal membrane preparation. Under an in vitro system, low concentrations of melatonin (1 pM–100 nM) potentiated the amplitude and frequency of spontaneous contractile responses in caecal rings and strips. This effect of melatonin was prevented by preincubation of the muscle preparation with a specific inhibitor of tyrosine kinase genistein (2 ÌM). In contrast to melatonin, agents such as epinephrine (10 nM), and isoproterenol (10 nM), which are known to increase the level of the intracellular cAMP and thereby activating protein kinase A, or dibutyryl-cAMP (1 mM), completely blocked the spontaneous contractions. The inhibitory influence of ß-agonists on contractions in caecum rings was partly reversed by the application of melatonin (0.1–1 ÌM). Epidermal growth factor (EGF) (0.2 Ìg/ml) which is linked to receptor-dependent tyrosine kinase, similar to melatonin, also restored the isoproterenol-induced suppression of spontaneous contractions. Neither melatonin nor EGF was able to modify isoproterenol-induced inhibition in genistein-treated caecum rings. It is concluded that melatonin has an anti-adrenergic action on the quail smooth muscle and this seems to be a receptor-mediated response. Moreover, the effect may be coupled to an activation of tyrosine kinase, which is likely to antagonize the influence of protein kinase A.