Increased heart microsomal Na(+) K(+)-transporting ATPase activity by tetrandrine in spontaneously hypertensive rats

In spontaneously hypertensive rats (SHR), the heart microsomal Na+, K(+)-ATPase showed a reduced activity with a higher Ca2+ sensitivity and lower responsiveness to K+. After SHR were treated with tetrandrine (Tet, ig 30 mg.kg-1, qd x 30 d) or when the microsomes obtained from SHR hearts were incubated with various concentrations of Tet, the myocardial Na+,K(+)-ATPase showed a higher activity with a decreased Km for K+, or ATP and an increased Vmax for K+. The in vitro stimulatory effect of Tet on the enzyme was dependent on the Na+ concentration in medium and was not abolished by CaCl2 1 mmol.L-1 or ouabain 0.1 mmol.L-1. However, when given in vivo, Tet tended to reduce the tolerance of the enzyme to Ca2+ and failed to change the responsiveness of the enzyme to ouabain. In vitro, Tet had similar but weaker stimulatory effect on the Na+,K(+)-ATPase from Wistar Kyoto rats.