Effects of muscle contraction and of adenosine on capillary transport and microvascular flow in dog skeletal muscle.

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Abstract
The postarteriolar response of capillary transport and microvascular flow distribution to muscle contraction and to adenosine was measured by the indicator dilution technique in isolated dog gracilis muscles perfused with blood at controlled flows. A model of dual circulation was used to analyze the partition of microvascular flow. The extraction (E) of 125I-iodoantipyrine (IAp) served as an indicator of capillary flow whereas the capillary transport capacity coefficient (PSc) of 22Na was used to assess the changes in capillary surface area available for exchange. Muslce contraction produced by electrical stimulation of the motor nerve increased mean E-IAp from 0.94 +/- 0.03 (sd) to 0.95 +/- 0.01 and produced a 2.0- to 2.9-fold increase in PSc-Na. Intra-arterial adenosine produced results similar to those caused by muscle contraction. We conclude that (1) in resting muscle, most of the flow circulates through exchanging blood vessels and (2) in addition to the primary mechanisms of arteriolar vasodilatation, a substantial increase in the number of capillaries available for exchange of materials plays an important role in the adaptive response to increased metabolic demand.