title: The effects of Ca2+ and Sr2+ at different modes of activation in the smooth muscle of the rat portal vein.
abstract: The dependence of Ca2+ for different modes of activation has been investigated in the rat portal vein in vitro by evaluating the effects of substituting Sr2+ for Ca2+ in the extracellular fluid. For the spontaneous contractile activity as well as for contractures induced by depolarizing (high-K+) solution Sr2+ can to a large extent replace Ca2+. Sr2+ is however a poor Ca2+-substitute for the contractile response induced by alpha-stimulation (noradrenaline). The inhibition of spontaneous contractions by beta-stimulation (isoprenaline) is markedly less pronounced in Sr2+ solutions. Available data in the literature suggest that agonist interaction with alpha-receptors, in contrast to beta-receptors, is Ca2+ independent. Our results thus suggest that there exists a Ca2+ specific step between the alpha-receptor agonist occupancy and membrane excitation.
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D011169:Portal Vein / Q000187:drug effects / Q000294:innervation; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013324:Strontium / Q000494:pharmacology*
chemical_list: D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D007545:Isoproterenol; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine; D013324:Strontium