CORRELATION BETWEEN RESPONSE TO NOREPINEPHRINE AND REMOVAL OF CA-45 FROM HIGH-AFFINITY BINDING-SITES BY EXTRACELLULAR EDTA IN RABBIT AORTIC SMOOTH-MUSCLE

  • 1 January 1979
    • journal article
    • research article
    • Vol. 211  (2) , 353-359
Abstract
Divalent ion chelators (EDTA, EGTA [ethylene glycol bis tetraacetic acid]) removed 45Ca from high-affinity binding sites in isolated rabbit aortic smooth muscle in a concentration-dependent manner. Measurement of 14C-labeled EDTA uptake (10-60 min incubation periods) yielded tissue/medium ratios equivalent to the extracellular space [14C]-sucrose tissue/medium ratio. Addition of high EDTA concentrations (0.5-1.5 mM) elicits large but reversible increases in 45Ca efflux, rapidly removes virtually all 45Ca from muscles previously incubated with 45Ca for as short a time as 7 min (filling of extracellular and superficial sites) or as long as 180 min (more complete equilibration of 45Ca), and blocks the norepinephrine-induced contractile response (this response is only partially inhibited by 0.05 mM EDTA, a concentration too low to remove most of the 45Ca present). Phosphatidyl serine, a compound known to decrease exchangeability of 45Ca in this tissue, inhibits the EDTA-induced increase in 45Ca loss. Those slowly depleted Ca2+ stores that are important for the contractile action of norepinephrine are removed by EDTA even though this chelator is confined to the extracellular space. Possibly, release or removal of high-affinity Ca2+ is regulated by EDTA-accessible Ca2+ bound at relatively superficial membrane sites.