Contribution of shrinkage of extracellular space to extracellular K⁺ accumulation in myocardial ischaemia of the rabbit

1. The contribution of the concentrating effect due to shrinkage of the extracellular space (ECS) to cellular K⁺ efflux on extracellular potassium ([K⁺]₀) accumulation in response to ischaemia was investigated in an isolated, blood-perfused rabbit papillary muscle preparation with a confined extracellular space. 2. The ECS was quantified using either of two extracellular markers, choline or tetramethyl ammonium (TMA), each with specific ion-selective electrodes, as well as by measurement of extracellular resistance (r₀). [K⁺]₀ and [Na⁺]₀ were also measured simultaneously using K⁺-and Na⁺-selective electrodes. 3. During ischaemia, [K⁺]₀ increased 3-fold from 4.2 ± 0.1 to 12.6 ± 1.0 mM at 10 min (n = 10) analogous to changes in the ischaemic heart in vivo. The ECS decreased to 83.9 ± 3.2% of control measured using 1 mM choline extracellularly (n = 9, P < 0.01) or to 85.7 ± 0.7% of control using 1 mM TMA (n = 6, P < 0.01). Nearly identical decreases in r₀ (84.1 ± 2.4%, n = 15, P < 0.01) occurred simultaneously. 4. The small decrease in the ECS contributed only 0.8-0.9 mM to the total increase in [K⁺]₀ of 8.4 mM and had a minor effect on transmembrane K⁺ flux. No significant differences between the relative changes in [choline] and [Na⁺]₀ were observed. This excluded a major transmembrane Na⁺ movement during early ischaemia. 5. Bumetanide (10 mM), an inhibitor of K⁺-Cl^- cotransport, a process which is involved in cell volume regulation consequent to osmotic cell swelling, significantly attenuated the increase in [K⁺]₀ after 6 min of ischaemia (8.3 ± 0.6 mM, n = 5 vs. 10.3 ± 0.4 mM in the control group, n = 6, P < 0.05), whereas N-ethylmaleimide (1 mM), a stimulator of this cotransporter, augmented [K⁺]₀ accumulation (12.0 ± 0.6 mM at 6 min, P < 0.05). 6. We conclude that during early myocardial ischaemia, a major component of [K⁺], accumulation is not caused by diminution of ECS per se, but rather by increased net K⁺ efflux due in part to K⁺-Cl^- cotransport secondary to myocyte volume regulation.