EVIDENCE FOR THE EXISTENCE OF 2 DISTINCT POOLS OF INTRACELLULAR CALCIUM IN THE RAT AORTA ACCESSIBLE TO MOBILIZATION BY NOREPINEPHRINE

  • 1 January 1982
    • journal article
    • research article
    • Vol. 221  (1) , 7-13
Abstract
The contractile responses of isolated rat thoracic aortic strips to norepinephrine (NE) and KCl in the absence of extracellular Ca were studied using 1,2-bis-(2-dicarboxymethylaminoethoxy)ethane (EGTA) as a Ca 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 but short-lived, with the magnitude of the maximum tension produced in Ca-free medium (containing 1 mM EGTA) being .apprx. 26% of that induced by NE in the presence of extracellular Ca. This phasic component could not be elicited a 2nd time in the same tissue unless the tissue was re-exposed to extracellular Ca. The 2nd component of the NE response in Ca-free medium was a slowly developing and sustained contraction which represented 24% of the maximum contraction obtainable in Ca-containing medium. The sustained contraction could be repeatedly elicited in the same tissue even after 5 continuous h in Ca-free EGTA medium. The 2 components of the biphasic response are dissociable and each is apparently dependent upon the mobilization of a distinct intracellular pool of Ca. A single initial mechanism appears to trigger the biphasic response to NE, with a probable involvement of the .alpha. adrenergic receptor.