Calcium-dependent fluxes of potassium-42 and chloride-36 during norepinephrine activation of rat aorta.

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Abstract
This study was designed to determine whether alpha-receptor-stimulated monovalent ionic fluxes in rat aorta required calcium, and, if so, whether both extracellular calcium and cellularly stored calcium are active. Calcium removal in the presence of 10 mM magnesium (to maintain membrane stability) inhibited the norepinephrine-stimulated increase in potassium-42 and chloride-36 efflux. However, the norepinephrine-stimulated increase in sodium-24 influx was relatively resistant to calcium depletion. Protocols were designed to measure the time course for the changes in potassium-42 efflux and contraction when calcium was removed or replaced in the presence of norepinephrine. The dose-dependent effect of a calcium antagonist (diltiazem) was also measured. A close correlation (r = 0.94) was found between inhibition of contraction and potassium-42 effluxes which followed the regression: % potassium-42 response = 1.0 X (% contraction) + 1.8%). The slope of 1.0 and intercept near zero suggests the hypothesis that norepinephrine-stimulated potassium-42 efflux and contraction are codependent on cellular calcium concentration. This co-dependence held for short phasic responses (approximately 1 minute), as well as longer tonic responses (greater than or equal to 5 minutes). It appears that calcium-dependent potassium-42 effluxes can be supported by both the influx of extracellular calcium and release of cellular stores. It is concluded that calcium-dependent potassium channels (and possibly chloride channels) are operative in rat aorta and are an important component of the graded membrane response to norepinephrine. The sodium channels, however, do not appear to share this same calcium dependency.