Effects of norepinephrine, dopamine and potassium on tension and 45Ca fluxes in canine and rabbit renal arteries.

The action and interactions of three vasoconstrictors, norepinephrine (NE), dopamine (DA) and elevated potassium ion (K+) on contractile responses and associated 45Ca movements were investigated in isolated rabbit and canine renal arteries (RA). Dose-response curves indicate that NE is 39 times more potent than DA in canine RA and 122 times more potent in rabbit RA. Prior exposure to 80 mM K+ did not prevent contractile responses to NE or DA but, conversely, K+ -induced responses did not occur after exposure to NE or DA. Responses to NE persisted after maximum DA-induced contractions but only a small response to DA was observed after a maximum NE-induced contraction. After 30 to 60 min in either a O-Ca or a O-Ca plus 0.05 mM EDTA solution, contractile responses were differentially inhibited (K+ more than DA more than NE). Efflux of 45Ca into a O-Ca plus EDTA solution was qualitatively similar in canine and rabbit RA. Addition of K+, DA or NE decreased the rate of 45Ca efflux in both RA; phentolamine abolished the NE-induced decrease and had no effect on the K+-induced decrease. The observed decrease in 45Ca efflux may reflect an inward shift of 45Ca from membrane binding sites. The mechanisms by which this effect is obtained appear to differ for K+, DA and NE. The differing actions of NE and DA cannot be explained solely by variations in potency at a singel type of receptor.