| Research Abstract |
1. lsolated hearts were perfused in Krebs-Henseleit buffer containing 0, 1.2 or 5mM Pi. lncreasing Pifrom 0 to 5mM significantly decreasedtotal [Mg^<2+>]i from 17.0 to 16.2mM and resulted in no significant change in [Pi]i. From the citrate/isocitrate ratio, free [Mg^<2+>]i was 0.36, 0.44 and 0.44mM for 0, 1.2 and 5mM Pi. The caculated intracellular pH was 6.98, 7.19 and 7.15 respectively. As Pi was increased from 0 to 5mM, [Cl^-]i decreased from 51.7 to 11.6mM.[K^+]i was invariant at 128 to 132mM giving a membrane potential of -83mV.[Na^+]i with increasing Pi was 1.62, 3.68 and 2.03mM.The cytosolic [SIGMAATP]/[SIGMAADP]/[SIGMAPi] decreased from 9,300 to 3,360 M^<-1> in the 0 and 5mM Pi perfused hearts. The free eergy of SIGMATP hydrolysis to SIGMAADP and SIGMAPi was found to be -13.1, -13.3 and -13.2Kcal/mole for 0, 1.2 and 5mM Pi hearts. The energy of the plasma membrane gradients were +12.7, +11.3 and +13.2Kcal/mole, or 97, 85 and 99% of the cellular phosphorylation potential. The extent and direction of the pasma membrane gradients in rat hearts were determined by cellular phosphorylation potential.
The effect of changing extracelular free Mg on cardiac function and Ca content was studied in working rat hearts. Total [Ca^<2+>]i decreased linearly from 0.85 to 0.43mM and [Mg^<2+>]i/[Ca^<2+>]i increased from 22.5 to 53.2 as [Mg^<2+>]o increased from 0 to 2mM.Cardiac O_2 consumpton and mitochondrial [NAD^+]/[NADH] were constant in each condition of [Mg^<2+>]o. The maximal cardiac efficiency was attained at a physiologically normal [Mg^<2+>]o of 0.5mM.
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