Potentiating and depressive effects of ouabain and potassium-free solutions on rat mesenteric resistance vessels.

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Abstract
We have investigated the in vitro effects of ouabain and K-free solutions on some pharmacological and electrophysiological properties of rat mesenteric resistance vessels (internal diameter approximately 190 micrometers). Vessels were mounted as ring preparations on a myograph capable of measuring their isometric wall tension. In normal saline solutions, vessels did not exhibit any tone and had a membrane potential of -54 mV. Both 1 mM ouabain and K-free solutions caused a transient depolarization of 5-8 mV; thereafter the membrane slowly depolarized to about -45 mV after 30 minutes. There was no mechanical response to ouabain, but K-free solutions caused a transient development of tension which could be inhibited by phentolamine (1 microM). In norepinephrine-activated vessels, exposure to ouabain or K-free solutions caused a small depolarization and an increase in tension. Long-term (30-minute) exposure to 1 mM ouabain or K-free solutions reduced the amplitude of norepinephrine responses and, for the lower (but not the higher) norepinephrine concentrations, the membranes were about 14 mV more depolarized than control. The mechanical responses to a cocktail of norepinephrine in a high potassium solution were, however, unaffected. Re-exposure to normal saline solution produced a transient hyperpolarization and transiently eliminated the norepinephrine response, but thereafter the membrane potential and response returned to normal. The results indicate that ouabain and K-free solutions can have both short-term potentiating and long-term depressive effects on the mechanical response of rat mesenteric resistance vessels to norepinephrine.

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