FORCE REGULATION BY CA2+ IN SKINNED SINGLE CARDIAC MYOCYTES OF FROG

Atrial and ventricular myocytes 200 to 300 mu m long containing one to five myofibrils are isolated from frog hearts. After a cell is caught and held between two suction micropipettes the surface membrane is de stroyed by briefly jetting relaxing solution containing 0.05% Triton X -100 on it from a third micropipette. Jetting buffered Ca2+ from other pipettes produces sustained contractions that relax completely on ces sation. The pCa/force relationship is determined at 20 degrees C by pe rfusing a closely spaced sequence of pCa concentrations (pCa = -log[Ca 2+]) past the skinned myocyte. At each step in the pCa series quick re lease of the myocyte length defines the tension baseline and quick res tretch allows the kinetics of the return to steady tension to be obser ved. The pCa/force data fit to the Hill equation for atrial and ventri cular myocytes yield respectively, a pK (curve midpoint) of 5.86 +/- 0 .03 (mean +/- SE; n = 7) and 5.87 +/- 0.02 (n = 18) and an n(H) (slope ) of 4.3 +/- 0.34 and 5.1 +/- 0.35. These slopes are about double thos e reported previously, suggesting that the cooperativity of Ca2+ activ ation in frog cardiac myofibrils is as strong as in fast skeletal musc le. The shape of the pCa/force relationship differs from that usually reported for skeletal muscle in that it closely follows the ideal fitt ed Hill plot with a single slope while that of skeletal muscle appears steeper in the lower than in the upper half. The rate of tension rede velopment following release restretch protocol increases with Ca2+ > 1 0-fold and continues to rise after Ca2+ activated tension saturates. T his finding provides support for a strong kinetic mechanism of force r egulation by Ca2+ in frog cardiac muscle, at variance with previous re ports on mammalian heart muscle. The maximum rate of tension redevelop ment following restretch is similar to twofold faster for atrial than for ventricular myocytes, in accord with the idea that the intrinsic s peed of the contractile proteins is faster in atrial than in ventricul ar myocardium.