Decreased activity of (Ca 2+ + Mg 2+)-adenosine triphosphatase (ATPase) and a hormone-specific defect in insulin regulation of ATPase in kidney basolateral membrannes from obese [formula omitted] rats

Abstract

The plasma membrane enzyme (Ca 2+ + Mg 2+)-adenosine triphosphatase (ATPase) is hormonally regulated and may participate in Ca 2+ signaling by removing excess Ca 2+ from the cell. Therefore, observations of a hormone-specific loss of insulin stimulation of ATPase in kidney membranes from non-insulin-dependent diabetic (NIDDM) rats may reflect their insulin-resistant state. Consequently, to evaluate whether additional insulin-resistant conditions are associated with impaired function of ATPase and with loss of regulation of the enzyme by insulin, studies were extended to investigate (Ca 2+ + Mg 2+)-ATPase activities and hormonal regulation of the enzyme in kidney basolateral membranes from obese and lean Zucker rats. (Ca 2+ + Mg 2+)-ATPase activity was lower in membranes from obese rats compared with lean rats. Maximal velocity (Vmax) of the enzyme activity was 29.2 ± 2.6 nmol P i/mg/min in obese rats versus 57.2 ± 6.5 in lean rats ( P < .05). However, the affinity of the enzyme for Ca 2+ was similar in obese and lean rats ( K m Ca 2+, 0.23 ± 0.025 v 0.23 ± 0.032 μmol/L Ca 2+). Also, the K m for ATP of the enzyme was similar in membranes from obese and lean rats. Insulin, parathyroid hormone (PTH), and cyclic adenosine monophosphate (cAMP) stimulated the ATPase activity in membranes from lean rats in a dose-dependent manner (15% to 28%). Also, the protein kinase C (PKC) stimulator 12- O-tetradecanoyl phorbol-13-acetate (TPA) increased the ATPase activity in membranes from lean rats. However, whereas PTH and cAMP maintained their stimulation of the enzyme also in membranes from obese rats, insulin and TPA lost their stimulatory effects on the enzyme in membranes from obese animals. Thus, a hormone-specific defect in insulin regulation of ATPase also occurs in obesity and may be related to abnormal metabolism of PKC in obesity.