Permeability of single muscle capillaries to potassium ions

A frog muscle preparation suitable for capillary micropuncture is described. The K⁺ permeability of single muscle capillaries was measured using the methods developed for frog mesenteric capillaries (C. Crone, J. Frøkjaer-Jensen, J. J. Friedman, and O. Christensen (1978), J. Gen. Physiol.71, 195–220). The K⁺ permeability of arterial capillaries was 8.6 × 10^?5 cm/sec (SD = 1.8; n = 7). The initial K⁺ permeability of venous capillaries was 13.1 × 10^?5 cm/sec (SD = 4.3; n = 6). In venous capillaries—unlike in arterial capillaries—a gradual increase in permeability associated with endothelial gap formation was observed in repeated measurements on the same capillary segment. This inflammatory response to exposure, micromanipulation, or perfusion could be partially blocked by pretreatment with promethazine. The study shows that frog muscle capillaries differ markedly from frog mesenteric capillaries, being 5–10 times less permeable to potassium ions although both belong to the category of continuous capillaries. The results demonstrate that whole-organ and single-capillary techniques for studying muscle capillary permeability yield values which comply within a factor of about 2.