Cardiac sarcoplasmic reticulum characteristics in hypertrophic hearts from spontaneously hypertensive rats

Summary Sarcoplasmic reticulum (SR) from left ventricles of rats that developed spontaneous hypertension was studied in vitro. Similar increases of left ventricular mass were found when grouping the animals into mild and severe hypertensives (average systolic arterial pressure of 168±4 and 202±6 mm Hg, respectively). The amount of SR protein (mg/g of left ventricle) was higher when obtained from hypertrophic ventricles of both hypertensive groups than from ventricles of the control group. The result agreed with the enhanced Ca²⁺ uptake exhibited by left ventricular homogenates of the hypertensive groups. Consequently, Ca²⁺ uptake in SR microsomes isolated per gram of left ventricle (nmol Ca²⁺/g muscle) was 51.62±10.06 and 64.99±12.84 in mildly and severely hypertensive groups vs. 17.37±5.79 in the control group (P<0.05). The SR microsomes obtained from ventricles of hypertensive rats showed an enhanced Ca²⁺ activated ATPase activity that was not accompanied by increased Ca²⁺ uptake at saturating calcium concentrations, but by increased affinity for calcium (K'_app of 1.09±0.28 and 2.67±0.16 M in SR microsomes of hypertrophic and control ventricles respectively; P<0.05). The rats of calcium loss, measured in SR vesicles passively loaded with⁴⁵Ca, were similar when assayed in SR obtained from ventricles of both hypertensive and normal rats. These results enable us to suggest that in hearts of rats presenting spontaneous hypertension, the function of the SR system could account for a normal handling of cytosolic calcium. They might support the absence of mechanical alterations described in hearts of young rats of the SHR strain.This work was supported by grants from the Consejo nacional de Investigaciones Científicas y Tecnicas de la República Argentina (CONICET).