Some ectothermic vertebrates are especially tolerant to anoxia and it is has been hypothesized that this requires a decrease in the number or the activity of ion channels in order to reduce energy consumption by ATP-dependent ion pumping (channel arrest hypothesis). Several studies have provided indirect evidence in favour of the channel arrest hypothesis; however, only a limited number of experiments have examined the activity of ion channels directly from animals exposed to long-term hypoxia or anoxia in vivo.

Using the whole-cell and cell-attach single-channel patch-clamp methods, we compared the inward rectifying K⁺ current (I_K1), which is the primary current affecting resting membrane potential, in isolated cardiac myocytes of normoxic and hypoxic crucian carp, an anoxia-tolerant fish. Crucian carp (N = 68) were kept in large aerated tanks (control) or in 2 l Erlenmeyer bottles from which oxygen was removed with N₂ gassing (hypoxic). The duration of hypoxia varied from 4 to 28 days. Electrophysiological experiments were made with freshly isolated ventricular myocytes at the acclimation temperature (5°C) of the fish (fish were humanely killed). All experiments were conducted with the permission of the local committee for animal experimentation and comply with the guidelines of animal experimentation in Finland.

Severe hypoxia (< 0.4 g l^-1 O₂) from 4 days to 28 days in duration did not have any effect on I_K1. Whole-cell conductance of I_K1 was 0.6 ± 0.05 nS pF^-1 (mean ± S.E.M.) in normoxic fish and did not change during the 4-week hypoxic period (cells isolated every fourth day; n = 17 or 18). Over the same time period Na⁺-K⁺-ATPase activity decreased 33 % (P < 0.03, unpaired t test; n = 8). Single-channel conductance was unchanged by hypoxia, being 20.5 ± 0.8 pS (n = 17) in control fish and 21.4 ± 1.1 pS (n = 11) in hypoxic fish when symmetric 140 mM K⁺ solutions were used. Furthermore, the open probability of the channel was unchanged being 0.80 ± 0.03 and 0.74 ± 0.04 in control and hypoxic fish, respectively (n = 18). Additionally open and closed times had identical distributions in normoxic and hypoxic fish.

These results suggest that the inward rectifier K⁺ channels are not modified by severe hypoxia in ventricular cardiac myocytes of the crucian carp. We conclude that crucian carp cardiac myocytes obtain energy savings through ‘spike arrest’ due to hypoxic bradycardia rather than via channel arrest.

This study was supported by the Academy of Finland (project #53481).

All procedures accord with current National guidelines.