Microvascular hematocrit and permeability-surface area product in contracting canine skeletal muscle in situ.

The dynamics of the microvasculature of the blood-perfused canine gastrocnemius-plantaris muscle in situ at rest and during contraction were determined using multiple-indicator dilution analysis. Permeability-surface area product (PS) was estimated using a bolus indicator dilution of 86Rb, with 125I-albumin serving as the reference tracer, while microvascular hematocrit (Hmv) was estimated using the relationship between plasma (125I-albumin) and erythrocyte (51Cr) tracer washout curves. The muscle was stimulated to contract, under self-perfusion, for 3-min periods with either isometric twitch (1.5, 3, or 5 Hz; 4 ms) or tetanic (20, 40, or 60 trains/min, 200 ms, 100 Hz) contractions, separated by 25 min of rest, randomized to prevent ordering effects. At all stimulation frequencies, Hmv increased significantly from rest value of 36.5 +/- 1.6% to 3 min of either isometric twitch or tetanic contractions. PS rose significantly from 0.08 +/- 0.002 ml/g min at rest to a maximum of 0. 40 +/- 0.01 ml/g min at 60 isometric tetanic contractions per minute and 0.38 +/- 0.01 at 5 Hz. Changes in PS appeared related to stimulation frequency in both twitch and tetanic contractions. The change in Hmv with muscle contraction appeared to depend on contraction frequency during twitch contractions only, but was independent of stimulus frequency during tetanic contractions.