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

Abstract

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_Kdescribing the backward reaction E₁+ 2K⁺→ E₂(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 ϵ-isoform of protein kinase C (ϵPKC) mediates effects of angiotensin II on the pump, we included specific ϵPKC-blocking peptide in patch pipette filling solutions. The peptide reversed cholesterol-induced pump inhibition.