Cardiovascular Contraction in Spontaneously Hypertensive Rat : Ca²⁺ Interaction of Myofibrils and Subcellular Membrane of Heart and Arterial Smooth Muscle

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Cardiac myofibrils isolated from the hearts of Okamoto-Aoki hypertensive rat (SHR) and normotensive Wistar rat (NR) showed essentially the same ATPase activity either in the absence or presence of Ca²⁺. On the other hand, Ca²⁺ activated ATPase activity of relaxed preparation of cardiac myofibrils from SHR was significantly lower than that from NR. ATPase activity and Ca²⁺ binding ability of cardiac sarcoplasmic reticulum (cardiac SR) and arterial subcellular membrane (arterial SR) isolated from SHR and NR were measured. The maximum Ca²⁺ binding capacity of cardiac SR from SHR (8.80±0.33 nmoles/mg) was significantly lower than that from NR (11.95±0.55). Also, cardiac SR from SHR had significantly elevated Ca²⁺ activated ATPase activity (0.171±0.009 μmoles/mg/min) compared with that of NR (0.100±0.009). And, the maximum Ca²⁺ binding capacity of arterial SR from SHR (23.55±0.72 nmoles/mg) was significantly lower than that from NR (39.35±4.89). ATPase activity of arterial SR from SHR (2.94±0.18 μmoles/mg/min) was significantly elevated compared with that of NR (2.22±0.14). Reduction of Ca²⁺ binding capacity of both cardiac and arterial SR may result in increase of intracellular free Ca²⁺ concentration which probably be one of the important factor for increase of muscular tone. If this abnormal intracellular distribution of Ca²⁺ is associated with the vascular smooth muscle, it would cause the increase of vascular resistance resulting in hypertension.