EFFECTS OF TERBUTALINE ON ALPHA-ADRENERGIC RESPONSES AND CA2+ INFLUX IN ISOLATED RABBIT AORTA

Effects of terbutaline applied in vivo or in vitro on a-adrenergic rec eptors in the rabbit aorta in normal and Ca2+-free solution, and on ba sal, high potassium-, and phenylephrine-stimulaled Ca2+ uptake into ao rta were investigated. Three day terbutaline administration (25 mg/kg, subcutaneously three times daily) to rabbits increased the pK, for ph entolamine in aorta rings (control 7.3 +/- 0.2, n = 9; terbutaline 7.8 +/- 0.2, n = 15). It also depressed phenylephrine-stimulated contract ions of aorta rings in Ca2+-free but not those in normal Krebs solutio n. It did not significantly depress the basal, or phenylephrine-evoked Ca2+ influx into aorta rings, but decreased high potassium-induced Ca 2+-influx (control 0.58 +/- 0.05 mu moles/g aorta; n = 3, terbutaline 0.41 +/- 0.06 mu moles/g aorta, n = 3). In vivo application of 50 mu M terbutaline did not significantly alter phenylephrine-stimulated cont ractions of aorta rings in Ca2+-free Krebs solution or significantly d epress basal or phenylephrine-induced Ca2+ influx into aortas, but did decrease high potassium-stimulated Ca2+-influx. Thus, 3-day terbutali ne administration increased the affinity of a-adrenergic receptors for phentolamine and had a tendency to increase contractions of aorta rin gs to phenylephrine. It also decreased high potassium-stimulated Ca2+- influx, and depressed phenylephrine-induced contractions in Ca2+-free Krebs solution, while in vitro terbutaline application also decreased potassium-induced Ca2+ influx.