Extracellular space and ionic distribution in rat ventricle.

POLIMENI, PHILIP I. Extracellular space and ionic distribution in rat ventricle. Am. J. Physiol. 227(3) : 676-683. 1974.-Extracellular space (ECS) of rat ventricle was measured by two independent methods: in vivo tracer ([35S]sulfatej distribution in the nephrectomized animal and by morphometric analysis of histological preparations fixed in isosmotic glutaraldehyde-saline. ECS determined by tracer and histological procedures in the same tissue were not significantly different. The extracellular compartment comprises 19.0 =t 0.2 9; of myocardial volume. Ventricular water content is 3.32 & 0.02 g/g dry wt, of which 0.82 rt 0.01 g (25ci;) consists of extracellular water. Electrolyte contents were measured in 16 samples (mmol/kg dry ventricle) : Na, 164 rt 3; K, 367 =I= 3; Cl, 125 rt 2; Ca, 3.59 zt 0.08; Mg, 44.5 =I= 0.4. Cellular concentrations (mmol/kg cell water) were calculated on the assumption of a homogeneous distribution throughout ECS: [Na] i = 16; [K] i = 144; [Cl] i = 13; [Ca] i = 0.6; [Mg] i = 17. When [Na] in the sarcoplasmic reticulum (SR) is assumed to equal [Na],, as suggested in studies of skeletal muscle, then calculated [Na]i is slightly greater than the chemical activity of cellular Na recently measured directly in the heart with ion-selective microelectrodes. Calculated [Cl] i is compatible with nonpassive distribution of cardiac Cl, unless Cl is also assumed to enter the SR.