Intracellular free Ca2+ and the hypercontracture of adult rat heart myocytes

The Ca2+ sensitivity of a population of isolated adult rat heart myocytes has been related to the Na+ content of the cells prior to Ca2+ exposure, and the intracellular free Ca2+ as reported by quin2 fluorescence when the cells are challenged with millimolar external Ca2+. Myocytes exposed to Ca2+ during quin2 loading show a resting intracellular free Ca2+ of 150 +/- 30 nM and retain the rod cell morphology of heart cells in situ. The myocytes take up Na+ and lose K+ when incubated in the cold in the absence of Ca2+. Large numbers of these rod-shaped, Na+-loaded myocytes hypercontract into grossly distorted round cell forms when exposed to physiological levels of Ca2+. The number of cells that hypercontract is proportional to the Na+ content of the cells prior to Ca2+ addition and can be directly related to the intracellular free Ca2+ concentration attained following Ca2+ addition. Fifty percent of the cells in a myocyte population hypercontract when the internal free Ca2+ concentration reported by quin2 reaches 400 nM and virtually all of the cells hypercontract when this value reaches 1 microM. The entry of Ca2+ into Na+-loaded myocytes is biphasic with one phase inhibited by Ca2+ channel blockade. This suggests that Ca2+ enters Na+-loaded myocytes by the Ca2+ channel as well as by Na+/Ca2+ exchange.