Rate-dependent effects of hypoxia on internal longitudinal resistance in guinea pig papillary muscles.

We have studied the independent and combined effects of 30 minutes' exposure to hypoxia and an increase in stimulation frequency from 0.5 Hz to 3.0 Hz on internal longitudinal resistance (ri) and conduction in guinea pig papillary muscles through the use of the voltage ratio method with air as the external insulator. Increasing stimulation frequency from 0.5 to 3.0 Hz in the presence of O2 caused no significant change in ri. Hypoxia to a level of PO2 = 30 mm Hg caused an increase in ri that averaged 13.7% at a stimulation frequency of 0.5 Hz and 46% at 3.0 Hz. In all experiments, the increase in ri during hypoxia at 3.0 Hz was greater than the increase at 0.5 Hz, but conduction velocity did not change at either rate. These results indicate that hypoxia causes rate-dependent cellular uncoupling but, under the conditions of our experiments, does not cause significant changes in conduction.