Comparison of degree of dependence of canine renal arteries and veins on high and low affinity calcium for responses to norepinephrine and potassium.

The relationships between tension responses to norepinephrine (NE) and high potassium (KCl) and high affinity 45Ca binding and mobilization were compared in canine renal arteries and veins. Contractile responses to 80 mM KCl were inhibited 83 (veins) and 88% (arteries) after respective 5-min and 60-min incubation periods in solutions with no added Ca++; responses to 10(-4) M NE were inhibited 87% (veins) and only 44% (arteries) after 60 min in the same solution. In solutions favoring low affinity binding of Ca++ (5.0 mM), uptake of 45Ca was similar in canine renal arteries and veins. However, in solutions favoring high affinity Ca++ binding (0.03 mM Ca++), 45Ca uptake was greater in arteries than in veins. Comparison of 45Ca efflux rates indicates a more rapid efflux and a greater Ca++-45Ca exchangeability in veins. After incubation with 45Ca and subsequent washout in an isosmotic (80.8 mM) La+++-substituted solution at 0.5 degrees C, 45Ca retention (residual Ca++ uptake) was proportionately greater in arteries than in veins at both high and low affinity sites. NE (10(-6) M) significantly increased La+++-resistant 45Ca uptake at both high and low affinity sites in veins but not in arteries. Thus, the NE-induced responses in veins depend upon extracellular Ca++ to a greater degree than in arteries. Even though veins have a lower number of high affinity Ca++ binding sites and this decreases the ability of the veins to retain sufficient Ca++ at these sites to maintain the response to NE, the responsiveness to NE appears to be maintained by an alternate mechanism, uptake of Ca++ at lower NE concentrations.