Sodium and calcium interactions in vascular smooth muscle cells of the rabbit ear artery.
The effects of Na-free and of K-free solutions on the membrane potential, on tension development, and on 45Ca exchange have been investigated in rabbit ear artery. The contraction induced by Na-free solutions and the tension which develops in K-free solutions after a delay of about 1 h are both submaximal. Exposure for 4 h to K-free solutions does not affect the membrane potential, whereas Na-free solutions depolarize the cells by 10-20 mV, depending on the Na-substitute. Neither the amplitude nor the rate constant of the slowly exchanging 45Ca-fraction is affected by these experimental procedures. Substituting external Na by choline or TMA induces a transient increase of the 45Ca-efflux rate which does not occur in a Ca-free efflux medium, and which can be blocked with La. K readmission to Na-enriched tissues hyperpolarizes the cells up to -100 mV and induces a relaxation, without exerting any effect on the 45Ca efflux rate. The release of Ca from intracellular stores, induced by histamine and FCCP, and its subsequent extrusion through the plasma membrane produce a transient stimulation of the 45Ca efflux, which is not affected by the reduction of the Na gradient. The transient contraction induced by histamine in Ca-free solutions is affected in a different way by different Na substitutes. The results do not fit the Na-Ca exchange hypothesis but are consistent with an effect of the Na gradient on the passive Ca influx.
D000818:Animals; D001158:Arteries / Q000502:physiology; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D004423:Ear / Q000098:blood supply; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects*; D009130:Muscle, Smooth / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology*; D011817:Rabbits; D012964:Sodium / Q000494:pharmacology*
D012964:Sodium; D011188:Potassium; D002118:Calcium