Abstract
Microsomal fractions prepared from rat ventricular muscle accumulated 23.15 ± 3.6 and 27.8 ± 2.4 nmol Ca 2+/mg microsomal protein when incubated for 2 and 5 min respectively in an oxalate-free incubation medium, and 263.2 ± 26.0 and 327.3 ± 48.6 nmol Ca 2+/mg microsomal protein when incubated for 2 and 5 min respectively in the presence of oxalate. These values for Ca 2+ binding (absence of oxalate) and uptake (presence of oxalate) are significantly ( P < 0.001) smaller than those obtained for guinea pig microsomal fractions. In the absence of Ca 2+ the rat microsomal fractions hydrolysed ATP at a rate which was significantly faster than that exhibited by the guinea pig microsomes. Doses of 1 and 5 m m caffeine increased the peak tension developed by guinea pig and decreased the peak tension developed by rat papillary muscles. These same doses of caffeine prolonged the time required to develop peak tension in both species, and showed the rate of Ca 2+ uptake. Doses of 1 and 5 m m caffeine slowed the rate at which the rat but not the guinea pig microsomes bound Ca 2+ (in the absence of oxalate). These results are interpreted to mean that there is a species variation in the Ca 2+-accumulating and ATPase activity of rat and guinea pig microsomal fractions.
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