Dietary cholesterol alters Na+/K+ selectivity at intracellular Na+/K+ pump sites in cardiac myocytes

A modest diet-induced increase in serum cholesterol in rabbits increases the sensitivity of the sarcolemmal Na+/K+ pump to intracellular Na+, whereas a large increase in cholesterol levels decreases the sensitivity to Na+. To examine the mechanisms, we isolated cardiac myocytes from controls and from rabbits with diet-induced increases in serum cholesterol. The myocytes were voltage clamped with the use of patch pipettes that contained osmotically balanced solutions with Na+ in a concentration of 10 mM and K+ in concentrations ([K+](pip)) ranging from 0 to 140 mM. There was no effect of dietary cholesterol on electrogenic Na+/K+ current (I-p) when pipette solutions were K+ free. A modest increase in serum cholesterol caused a [K+](pip)-dependent increase in I-p, whereas a large increase caused a [K+](pip)-dependent decrease in I-p. Modeling suggested that pump stimulation with a modest increase in serum cholesterol can be explained by a decrease in the microscopic association constant K-K describing the backward reaction E-1+2K(+)-->3E(2)(K+)(2), whereas pump inhibition with a large increase in serum cholesterol can be explained by an increase in K-K. Because hypercholesterolemia upregulates angiotensin II receptors and because angiotensin II regulates the Na+/K+ pump in cardiac myocytes in a [K+](pip)-dependent manner, we blocked angiotensin synthesis or angiotensin II receptors in vivo in cholesterol-fed rabbits. This abolished cholesterol-induced pump inhibition. Because the epsilon-isoform of protein kinase C (epsilonPKC) mediates effects of angiotensin II on the pump, we included specific epsilonPKC-blocking peptide in patch pipette filling solutions. The peptide reversed cholesterol-induced pump inhibition.