45Ca-efflux in embryonic chick heart and its modification by caffeine and ryanodine.

Ontogenic changes in the kinetics of exchangeable cellular calcium were studied in embryonic (ECV) and post-hatch (PHCV) chick ventricular tissue by monitoring 45Ca-efflux. The isolated whole ventricle (5 & 7 days ECV) or ventricular strips (12 & 18 days ECV and 1-2 days PHCV) were "loaded" with 45Ca (37 degrees C) and then passed through a series of tubes containing efflux solution (4 degrees C) to determine 45Ca-efflux. Curve 'peeling' of the efflux curve indicated existence of 3 kinetically distinct components of exchangeable cellular Ca2+ compartments: C1, C2 & C3. The size of C1, which was the largest in 5 & 7 days ECV decreased significantly to become minimum in 18 days ECV & PHCV. The rate constant of this compartment, however, reduced with the age of the embryo. In contrast, the size of C3 increased with the embryonic development to become the largest in 18 days ECV & PHCV. An increase in the rate constant of this compartment was also observed during embryogenesis. The size and rate constant of C2 remained unaltered during development. However, the increase in size of C3 during embryonic development indicates differentiation of Ca2+ storage sites, like sarcoplasmic reticulum (SR), during the later stages. Caffeine (10 mM) and ryanodine (10 microM) 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.(ABSTRACT TRUNCATED AT 250 WORDS)