Localization of rapidly exchangeable calcium in mammalian heart muscle

Two new methods were used to calculate net labeled Ca++ compartmental influx into rabbit myocardium: 1) continuous measurements of arteriovenous differences in 45Ca levels are corrected for longitudinal mixing and for varying transit times by a double-isotope technique with the relatively impermeant 131I-labeled albumin as the referent substance; 21 tritiated sucrose and 45Ca contents were measured in ventricular muscle after perfusion with the doubly labeled perfusate and washout of aqueous perfusate with fluorocarbon 80. Both methods yielded similar results. Only 52% of the Ca++ efflux from the vascular bed during the rapid flux period can be accounted for as Ca++ in interstitial water under control perfusion conditions. Contracture induced by substituting K+ for Na+ in the perfusate brings the fraction not accounted for as Ca++ in interstitial water from 48 to 60% of the total in 5 min. When irreversible contracture is induced by return to normal [Ca++] after short periods of excitation-contraction uncoupling due to zero [Ca++] perfusion, the fraction not accounted for as Ca++ in interstitial water is 68% in 5 min.