Capillary, Interstitial, and Cell Membrane Barriers to Blood-Tissue Transport of Potassium and Rubidium in Mammalian Skeletal Muscle

Blood-tissue transport of 42 K, 86 Rb, and 22 Na was studied in perfused gracilis muscles of dogs to ascertain the influence of capillary wall, interstitial space, and muscle cell membranes on overall transport kinetics. Single-injection and continuous-infusion techniques were used, with T-1824 or 51 Cr-hemoglobin as nondiffusible references. Results were as follows: (1) Extraction ( E ) of the diffusible solutes was incomplete even in the earliest samples of venous outflow. (2) E Rb equaled E K in the first few samples, but later, as both declined, the ratio of E Rb to E K became less than one; stable ratios between 0.6 and 0.8 were established in different experiments. (3) E Na was initially slightly less than E Rb or E K but declined much more sharply in successive samples. (4) Steadystate E Rb and E K decreased with increasing blood flow but maintained a constant ratio. Vasomotion sometimes changed E Rb and E K but had little effect on their ratio. Local anesthetics (procaine or cocaine) brought the ratio of E Rb to E K closer to one. Analysis in terms of capillary and interstitial barriers, which do not discriminate between K, Rb, or Na, in series with a muscle cell membrane barrier, which does, leads to the following conclusions. (1) For K, the nondiscriminating barrier (mainly capillary wall) offers about 70% of the total resistance to transport and the discriminating barrier (cell membrane) about 30%. (2) For Rb, each barrier offers half the total resistance. (3) The resting muscle cell membrane is 2.5 times more permeable to K than to Rb. Local anesthetics reduce cell permeability to both ions and decrease this ratio. (4) For Na, resistance at the capillary wall is about 1.3 times that for K or Rb. Resistance at the cell membrane is, of course, very much greater.