Functional involvement of α1and α2-adrenoceptors in86Rb efflux from liver slices and lipolysis in guinea-pig isolated adipocytes

Effects of verapamil and elgodipine on isoprenaline-induced metabolic responses in rabbits

Verapamil (0.17 microg kg(-1) min(-1) intravenous, i.v.) but not elgodipine (35 ng kg(-1) min(-1)) modestly enhanced the weak blood glucose increase induced by the i.v. infusion of isoprenaline (0.3 microg kg(-1) min(-1)) in conscious rabbits. However, elgodipine but not verapamil suppressed the increase in circulating insulin evoked by the agonist. Both drugs enhanced the rise in plasma lactate mediated by isoprenaline but only elgodipine potentiated the lipolytic effect of the agonist. In isolated islets elgodipine (10(-6) M) blocked forskolin (10(-6) M)-induced insulin release. However, in rabbit adipocytes elgodipine potentiated both glycerol release and cAMP accumulation induced by isoprenaline (10(-8)-10(-6) M). Excess K(+) (40-60 mM) did not alter basal lipolysis or the response to isoprenaline in either rabbit or mouse adipocytes. Therefore, Ca2+ influx through L-type Ca2+ channels does not seem to play a significant role in the lipolytic effect of isoprenaline. Metabolic alterations found with Ca2+ channel antagonists were of minor intensity and probably devoid of pathological implications.

Effects of catecholamines on plasma potassium: the role of alpha- and beta-adrenoceptors

The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha 1- and alpha 2-adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca(2+)-dependent-K(+)-channels. Stimulation of beta 1- and beta 2-adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na(+)-K(+)-ATPase. Beta 3-adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta 3-adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha-adrenoceptor agonist) and isoprenaline (beta-adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.