The role of extracellular sodium on heart muscle energetics

Abstract

A study has been made of changing external sodium concentration [Na]e, over the range 75 to 200 mmol X l-1, on contractile parameters and heat production in isolated, arterially perfused, interventricular rabbit septa.- The observed changes in maximum rate of contraction with [Na]e, either in the presence of a constant external Ca concentration [Ca]e or in the presence of a constant [Na]e2/[Ca]e ratio, paralleled those observed for tension development (T). On the other hand the maximal rate of relaxation (-Tmax) and the ratio -Tmax/T increased. While the ratio between active heat production and developed tension remained unaltered (0.111 +/- 0.003 mJ X mN-1 X g-1 dry weight), resting heat production increased with [Na]e2 with a slope of 95 +/- 18 mW X g-1 X mol-2 X l2. Under resting conditions, a decrease in [Na]e of 50 mmol X l-1 induced a fall in 42K uptake of about 16 nmol X s-1 X g-1 without changes in 42K efflux, suggesting that such an intervention depresses K influx. If the depressed K influx, induced by a decrease in [Na]e of 50 mmol X l-1, is associated with a decrease in Na-K pump activity, a fall in resting heat production of about 0.64 mW X g-1 would be expected. This represent 56% of the calculated change in the resting heat production, 1.14 +/- 0.22 mW X g-1 (mean +/- one confidence interval), suggesting that some process in addition to a depressed Na-K pump activity may be altered by changes in [Na]e.

Effects of caffeine, verapamil, lithium, and KB-R7943 on mechanics and energetics of rat myocardial bigeminies

“…Although it is purely speculative, it can be hypothesized that whereas lithium might be inhibiting the forward and reverse mode of the NCX, on the outside of the cell it will be competing with 145 meq Na ϩ /l and on the inside of the cell with perhaps Յ5 meq Na ϩ /l. This agrees with the previous finding that reduction of extracellular Na ϩ (which should particularly influence the NCX) increases the ratio of ϪdT/dt to T (i.e., the speed of relaxation normalized to the maximum tension developed during a contraction) (45).…”

“…The trans-SL inward movement of Ca 2ϩ during the action potential seems to be mediated by voltage-operated channels (23,37,45) and by the NCX (25,52). Depending on the Na ϩ and Ca 2ϩ electrochemical gradients, the NCX can operate in a Ca 2ϩ -influx mode (8,9,28).…”

Mitochondrial role in ischemia–reperfusion of rat hearts exposed to high-K+cardioplegia and clonazepam: energetic and contractile consequences

“…The calorimetric approach was widely used to estimate changes in exothermic cellular processes during cardiac activity and resting state by the thermopiles method (Loiselle 1987). In particular, online calorimetry for perfused tissues was extensively employed for studying the energetics of Ca 2+ movement in the steady beating heart (Ponce-Hornos et al 1982;Ponce-Hornos et al 1987;Ponce-Hornos et al 1992), under the transient conditions of a beat (Ponce-Hornos et al 1995;Consolini et al 1997;Ma ´rquez et al 1997;Bonazzola et al 2002), and also during isolated beats under ischemia and hypoperfusion (Consolini et al 2001). With this methodology, the role of mNCX and the effects of clonazepam on the postischemic recovery of a beating heart exposed or not to a high-K + cardioplegia were evaluated in this work.…”

Cardiac basal metabolism: energetic cost of calcium withdrawal in the adult rat heart

“…Energy expenditure by the quiescent myocardium includes the operation of active transport systems associated with the sarcolemma (SL), sarcoplasmic reticulum (SR) and mitochondria (MIT) which maintain ionic cell homeostasis. It has been shown that cardiac resting metabolism is affected by changes in extracellular ionic composition which alter the behaviour of the associated ionic transport systems (Ponce‐Hornos et al. 1987, 1992, Cooper et al.…”

“…Keywords basal metabolism, calcium depletion, calorimetry, mitochondrial Ca 2+ cycling.Energy expenditure by the quiescent myocardium includes the operation of active transport systems associated with the sarcolemma (SL), sarcoplasmic reticulum (SR) and mitochondria (MIT) which maintain ionic cell homeostasis. It has been shown that cardiac resting metabolism is affected by changes in extracellular ionic composition which alter the behaviour of the associated ionic transport systems (Ponce-Hornos et al 1987, Cooper et al 2001. Although changes in extracellular calcium concentration (Ca 2+ o ) in the submillimolar to millimolar range do not modify the energetic cost associated with cardiac…”