Calcium exchange, structure, and function in cultured adult myocardial cells

Cells digested from adult rat heart and cultured for 14 days demonstrate all the structural elements, in mature form, associated with the process of excitation-contraction (EC) coupling. The transverase tubular (TT) system is well developed with an extensive junctional sarcoplasmic reticulum (JSR). In nonphosphate-containing buffer contraction of the cells is lost as rapidly as zero extracellular Ca concentration ([Ca]o) solution is applied (less than 10 s) and a negative contraction staircase is produced on increase of stimulation frequency. Structurally and functionally the cells have the characteristics of adult cells in situ. 45Ca exchange and total 45Ca measurement in N-2-hydroxyethylpiperazine N'-2-ethanesulfonic acid (HEPES)-buffered perfusate define three components of cellular Ca: a rapidly exchangeable component (t1/2 less than 25 s) accounting for 36% of total Ca, a slowly exchangeable component (t1/2 53 min) accounting for 7% of total Ca, and the remaining 57% cellular Ca is "inexchangeable" (demonstrates no significant exchange within 60 min). The slowly exchangeable component can be increased 10-fold within 60 min by addition of phosphate to the perfusate. The Ca distribution and exchange characteristics are little different from those of 3-day cultures of neonatal rat heart previously studied [Langer, G. A., J. S. Frank, and L. M. Nudd. Am. J. Physiol. 237 (Heart Circ. Physiol. 6): H239-H246, 1979]. The results suggest that the cells are representative of adult cells in situ and that both sarcolemmal-bound and sarcoplasmic reticular Ca contribute to the component of Ca that is rapidly exchangeable.