CA-45 EFFLUX IN EMBRYONIC CHICK HEART AND ITS MODIFICATION BY CAFFEINE AND RYANODINE

Ontogenic changes in the kinetics of exchangeable cellular calcium wer e studied in embryonic (ECV) and post-hatch (PHCV) chick ventricular t issue by monitoring Ca-45-efflux. The isolated whole ventricle (5 & 7 days ECV) or ventricular strips (12 & 18 days ECV and 1-2 days PHCV) w ere ''loaded'' with Ca-45 (37-degrees-C) and then passed through a ser ies of tubes containing efflux solution (4-degrees-C) to determine Ca- 45-efflux. Curve 'peeling' of the efflux curve indicated existence of 3 kinetically distinct components of exchangeable cellular Ca2+ compar tments: C 1, C2 & C3. The size of C 1, which was the largest in 5 & 7 days ECV decreased significantly to become minimum in 18 days ECV & PH CV. The rate constant of this compartment, however, reduced with the a ge of the embryo. In contrast, the size of C3 increased with the embry onic development to become the largest in 18 days ECV & PHCV. An incre ase in the rate constant of this compartment was also observed during embryogenesis. The size and rate constant of C2 remained unaltered dur ing development. However, the increase in size of C3 during embryonic development indicates differentiation of Ca2+ storage sites, like sarc oplasmic reticulum (SR), during the later stages. Caffeine (10 mM) and ryanodine (10 muM) enhanced fractional escape rate during slow phase (ie 120-180 min) of efflux at all developmental stages. The magnitude of enhancement increased during later stages of development indicating greater prominence of SR with the age of embryo. It is concluded that : (i) Exchangeable Ca2+ in ECV is distributed in 3 compartments at all the stages of development, (ii) C1 is more prominent at earlier stage s (5 & 7 days ECV), and C3 at later stages (12 days ECV), and (iii) co ntribution of SR to Ca2+ storage increases with age of the embryo.