Size-dependent effects of microspheres on vasoconstrictor-mediated change in oxygen uptake by perfused rat hindlimb

There are two vascular flow routes in skeletal muscle that can be accessed by different vaso constrictors acting at selective sites in the vascular tr ee. Thus, angiotensin II (AII) and serotonin (5-HT), which stimulate and in hibit metabolism, do so by directing flow to nutritive and nonnutritive rou tes, respectively. In the present study the association between vascular fl ow route recruitment and metabolism was assessed by embolism with microsphe res of different sizes. Latex microspheres (MS) of four sizes, 5.4 (MS5), 1 1.8 (MS12), 23.4 (MS23), and 93.6 mum (MS94), were injected during AII- or 5-HT-mediated constriction or under basal conditions and the effects on hin dlimb oxygen uptake (VO2), perfusion pressure, and venous flow rate were de termined. MS5 or MS12 partially reversed 5-HT-mediated inhibition of VO2 by 39 and 55%, respectively (P < 0.05), fully reversed All-mediated stimulati on of VO2 (P < 0.05), stimulated basal VO2 (P < 0.05), and increased pressu re while only marginally (< 10%) decreasing venous flow. MS23 or MS94 dose- dependently increased pressure and inhibited VO2, during basal or 5HT- and All-mediated constriction, while only marginally decreasing venous flow. In conclusion, microspheres of less than 12 mum when injected into the consta nt flow perfused rat hindlimb can alter metabolism by altering flow distrib ution between nutritive and nonnutritive routes. Larger MS (greater than or equal to 24 mum) are nondiscriminating possibly because they exceed the si ze of vessels in which branch points to the two vascular routes are located . Overall the findings provide further evidence for two microvascular route s in muscle, one nutritive and the other nonnutritive, (C) 2001 Academic Pr ess.