A voltage-dependent noradrenaline-sensitive intracellular calcium store in smooth muscle cells of the guinea-pig aorta

In this study we investigated the effects of K⁺ depolarization on the contractile responses of guinea-pig aortic smooth muscle to noradrenaline (NA) and caffeine (CAF) in Ca²⁺-free Krebs solution (CafKS). NA (1 μM) or CAF (10 mM) in regular CafKS induced a (termed first) phasic contraction, but responses to a second NA or CAF stimulation were significantly reduced, suggesting a depletion of the common intracellular (NA/CAF sensitive) store previously reported by investigators. After the depletion of this store (i) neither 105 mM KCl CafKS nor 10 mM CAF in this high K⁺ solution induced contraction; (ii) NA-induced contractions were enhanced by KCl, increasing dose dependently with KCl ranging from 25 to 105 mM; (iii) in 105 mM KCl CafKS repetitive NA stimulations elicited enhanced and nondecreasing responses equal to or greater than the first contraction, suggesting an efficient recycling of intracellular Ca²⁺. In the presence of 10 μM cyclopiazonic acid NA-induced contraction was enhanced, but the response to a subsequent NA stimulation in the absence of cyclopiazonic acid was greatly reduced, implying an inhibition of the Ca²⁺ recycling function; (iv) 2.0 mM EGTA (ethylene glycol bis-(β-aminoethyl ether)-N,N'-tetraacetic acid) attenuated the NA-induced contractions in regular and 105 mM KCl CafKS. On the basis of the above results we propose that in the smooth muscle cells of guinea-pig aorta, there are two intracellular Ca²⁺ stores: a NA/CAF-sensitive store and a voltage-dependent NA-sensitive store with effective Ca²⁺ recycling capability.