The Role of Calcium in the Regulation of Mitochondrial Respiration under Various Substrate Combinations in the Heart and Kidney

Abstract

It has been recognized that the efficiency of mitochondrial oxidative phosphorylation (OxPhos) and energy metabolism depends on the choice of respiratory substrates, which feed electrons to the mitochondrial electron transport chain. In addition, mitochondrial enzymes, such as pyruvate dehydrogenase (PDH), isocitrate dehydrogenase (ICDH), and alpha‐ketoglutarate dehydrogenase (AKGDH), have been shown to be stimulated by calcium ions (Ca2+) in isolated enzyme experimental systems. However, the extent of Ca2+ stimulation of these enzymes and the resulting integrated stimulatory effect on the efficiency of mitochondrial OxPhos and energy metabolism in in‐situ and in‐vivo states in the heart and kidney under different substrate combinations are not well‐understood. In this study, we investigated the effect of a variety of substrate combinations and Ca2+ alternations that optimize mitochondrial respiration and ATP synthesis. We hypothesized that Ca2+ stimulation of mitochondrial respiration and ATP synthesis is both substrate dependent and tissue specific. To study mitochondrial function under a specific substrate combination and Ca2+, cardiac and renal mitochondria were isolated from Sprague‐Dawley rats, and oxygen consumption rates (JO2) were measured in the presence and absence of ADP that corresponded to leak (state 2) and ADP‐stimulated (state 3) states, respectively. We first measured the time courses of JO2 at various ADP concentrations to find the saturated ADP‐stimulated JO2. This saturated ADP concentration was then used to measure the time‐courses of JO2 under different [Ca2+]. The substrate combinations used include pyruvate + malate (PM), glutamate + malate (GM), alpha‐ketoglutarate + malate (AM), palmitoyl‐L‐carnitine + malate (PCM), and succinate + rotenone (SUC + ROT). Different substrates led to different levels of the saturated state 3 JO2. On average, it was 9 times higher in the heart compared to the kidney with the PM substrate. Also, the alterations in the respiratory substrates led to dramatically different respiratory rates/dynamics in the isolated mitochondria of the chosen organs. The results also demonstrated that Ca2+ significantly increased mitochondrial state 3 JO2 with GM and AM substrates compared to the other substrates in both organs (Table 1). Moreover, Ca2+ effects on mitochondrial respiration were found to be biphasic, i.e. a small increase in [Ca2+] had a stimulatory effect while large increases had inhibitory effects (Figure 1). These results suggest that energized isolated mitochondria in the presence of Ca2+ do not respond the same extent when different substrates are used. We conclude that isolated mitochondria require a suitable substrate combination, proper [Ca2+], and specific tissue source of mitochondria to achieve an optimal level of OxPhos and energy metabolism comparable to in‐vivo conditions.Support or Funding InformationThis work was supported by the NIH grants P01‐GM066730, P01‐HL116264, R01‐HL‐122662, and U01‐HL122199.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.