The vascular response to zinc varies seasonally: effect of pinealectomy and melatonin.

The effect of physiological concentrations of zinc on the in vitro pressor responses of male Wistar rat mesenteric preparations to norepinephrine was systematically studied over a 2-year period. A marked seasonal variation in the response of the preparations was observed over a range of zinc concentrations from 3.5 X 10(-11) to 3.5 X 10(-6) M. All concentrations of zinc inhibited pressor responses during the winter (November to March), but during the spring months (April to May) zinc became very much less effective and low concentrations of zinc potentiated the response. Inhibition of the pressor response at this time only occurred at zinc concentrations above 3.5 X 10(-7) M. In the summer months. (June to September) the zinc effect shifted back towards the winter situation with lower concentration of zinc potentiating and higher concentrations inhibiting vasoconstriction. Pinealectomy of the rats a week prior to preparation of the mesenteric beds very significantly enchanced pressor responses to norepinephrine in the winter months but had much less effect in the summer. The seasonal variation noted in preparations from control rats was maintained following pinealectomy but the amplitude was reduced. Perfusion of melatonin (4.3 X 10(-10) M through preparations from pinealectomized rats enhanced the inhibitory effect of zinc compared to the response to zinc from similarly perfused preparations from control rats. Thus, the rat mesenteric vascular bed is an in vitro model of seasonal variation in vascular reactivity which is sensitive to physiological concentrations of zinc. The origin of the seasonal rhythmicity remains speculative particularly in view of the fact that the rats were maintained under constant photoperiod. That the pineal may modulate the seasonal effect of zinc is suggested by the shift in response to zinc following pinealectomy. Melatonin reversed the effect of pinealectomy on this preparation implicating it as a possible mediator of normal vascular reactivity to zinc. A possible influence of magnetic radiation in evaluating the occurrence of seasonally varying phenomena in vitro should be considered.