Evidence for the Existence of Two Distinct Pools of Intracellular Calcium in the Rat Aorta Accessible to Mobilization by Norepinephrine

The contractile responses of isolated rat thoracic aortic strips to norepinephrine (NE) and KCl in the absence of extracellular calcium were studied using 1,2-bis-(2-dicarboxymethylaminoethoxy)ethane (EGTA) as a calcium chelating agent. Whereas aortic tissue which had been washed in EGTA-containing buffer was refractory to KCl, it remained biphasically responsive to stimulation by NE. The initial phasic contraction induced by NE was rapidly produced by short-lived, with the magnitude of the maximum tension produced in calcium-free medium (containing 1 mM EGTA) being approximately 26% of that induced by NE in the presence of extracellular calcium. This phasic component could not be elicited a second time in the same tissue unless the tissue was re-exposed to extracellular calcium. The second component of the NE response in calcium-free medium was a slowly developing and sustained contraction which represented 24% of the maximum contraction obtainable in calcium-containing medium. The sustained contraction could be repeatedly elicited in the same tissue even after 5 continuous hr in calcium-free EGTA medium. The two components of the biphasic response are dissociable and each is apparently dependent upon the mobilization of a distinct intracellular pool of calcium. However, a single initial mechanism appears to trigger the biphasic response to NE, with a probable involvement of the alpha adrenergic receptor.