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

Abstract

Blood-tissue transport of ⁴²K, ⁸⁶Rb, and²²Na 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 ⁵¹Cr-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.