Seasonal variation in responses of the toad renal vasculature to adrenaline

Adrenaline and noradrenaline produced both constriction and dilatation of the toad renal vasculature: constrictor effects were mediated by alpha-adrenoceptors; dilator effects were mediated by beta-adrenoceptors. Vasoconstriction was the predominant response to these amines. Dilatation was only revealed after blockade of alpha-adrenoceptors and constriction of the vasculature. There was a marked seasonal variation in the constrictor responses to adrenaline, but not to noradrenaline. The maximal increase in renal vascular resistance produced by adrenaline in summer was greater, by a factor of three, than the maximum constrictor response in winter. The response to adrenaline in summer was also significantly greater than the responses to noradrenaline in both summer and winter. However, after treatment with propranolol there was no difference between the maximum vasoconstrictions to these two amines, in summer or in winter. Determination of dissociation constants for phentolamine indicated that adrenaline and noradrenaline acted on the same population of alpha-adrenoceptors in both summer and winter. The enhanced vasoconstriction to adrenaline in summer appears to be due to a reduced potency of adrenaline on renal vascular beta-adrenoceptors. The reduction in potency may be caused by a subtle configurational change in the beta-adrenoceptors, perhaps induced by hormonal changes associated with the onset of breeding.