Chronic amiodarone-induced inhibition of the Na+-K+ pump in rabbit cardiac myocytes is thyroid-dependent: comparison with dronedarone.

OBJECTIVE: To examine the thyroid-dependence of the effect of amiodarone on the sarcolemmal Na(+)-K(+) pump, and the effect on the pump of dronedarone, a deiodinated amiodarone congener without influence on thyroid status. METHODS: New Zealand white rabbits underwent total thyroidectomy, sham thyroidectomy or thyroidectomy and concomitant oral amiodarone therapy. After 5 weeks, Na(+)-K(+) pump current was measured using the whole-cell patch-clamp technique in isolated ventricular myocytes. Pump current was also measured in myocytes isolated from a separate group of rabbits not subjected to thyroidectomy but treated with dronedarone, or placebo for 3 weeks. RESULTS: Treatment of thyroidectomised rabbits with amiodarone caused a significant prolongation of the corrected QT interval (QT(c)) and sinus cycle length. Na(+)-K(+) pump current measured in myocytes isolated from thyroidectomised rabbits was significantly lower than pump current in myocytes from sham-operated controls. However, treatment of thyroidectomised rabbits with amiodarone did not cause any additional decrease in pump current. Treatment with dronedarone caused prolongation of QT(c). However, it had no effect on Na(+)-K(+) pump current. CONCLUSIONS: The inhibitory effect of amiodarone on Na(+)-K(+) pump current is thyroid-dependent, whereas the effects on heart rate and QT(c) are at least partially mediated by thyroid-independent mechanisms. In contrast to its parent compound, dronedarone has no significant effects on the activity of the Na(+)-K(+) pump.

Alloxan-induced diabetes reduces sarcolemmal Na+-K+ pump function in rabbit ventricular myocytes.

The effect of diabetes on sarcolemmal Na(+)-K(+) pump function is important for our understanding of heart disease associated with diabetes and design of its treatment. We induced diabetes characterized by hyperglycemia but no other major metabolic disturbances in rabbits. Ventricular myocytes isolated from diabetic rabbits and controls were voltage clamped and internally perfused with the whole cell patch-clamp technique. Electrogenic Na(+)-K(+) pump current (I(p), arising from the 3:2 Na(+)-to-K(+) exchange ratio) was identified as the shift in holding current induced by Na(+)-K(+) pump blockade with 100 micromol/l ouabain in most experiments. There was no effect of diabetes on I(p) recorded when myocytes were perfused with pipette solutions containing 80 mmol/l Na(+) to nearly saturate intracellular Na(+)-K(+) pump sites. However, diabetes was associated with a significant decrease in I(p) measured when pipette solutions contained 10 mmol/l Na(+). The decrease was independent of membrane voltage but dependent on the intracellular concentration of K(+). There was no effect of diabetes on the sensitivity of I(p) to extracellular K(+). Pump inhibition was abolished by restoration of euglycemia or by in vivo angiotensin II receptor blockade with losartan. We conclude that diabetes induces sarcolemmal Na(+)-K(+) pump inhibition that can be reversed with pharmacological intervention.

Dependence of Na+-K+ pump current-voltage relationship on intracellular Na+, K+, and Cs+ in rabbit cardiac myocytes.

To examine effects of cytosolic Na+, K+, and Cs+ on the voltage dependence of the Na+-K+ pump, we measured Na+-K+ pump current (Ip) of ventricular myocytes voltage-clamped at potentials (Vm) from 100 to +60 mV. Superfusates were designed to eliminate voltage dependence at extracellular pump sites. The cytosolic compartment of myocytes was perfused with patch pipette solutions with a Na+ concentration ([Na]pip) of 80 mM and a K+ concentration from 0 to 80 mM or with solutions containing Na+ in concentrations from 0.1 to 100 mM and K+ in a concentration of either 0 or 80 mM. When [Na]pip was 80 mM, K+ in pipette solutions had a voltage-dependent inhibitory effect on Ip and induced a negative slope of the Ip-Vm relationship. Cs+ in pipette solutions had an effect on Ip qualitatively similar to that of K+. Increases in Ip with increases in [Na]pip were voltage dependent. The dielectric coefficient derived from [Na]pip-Ip relationships at the different test potentials was 0.15 when pipette solutions included 80 mM K+ and 0.06 when pipette solutions were K+ free.

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 (Ip) when pipette solutions were K+ free. A modest increase in serum cholesterol caused a [K+]pip-dependent increase in Ip, whereas a large increase caused a [K+]pip-dependent decrease in Ip. Modeling suggested that pump stimulation with a modest increase in serum cholesterol can be explained by a decrease in the microscopic association constant KK describing 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 KK. 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.