Nonshivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue as the dominant site of the calorigenesis induced by noradrenaline

Abstract

Cardiac output (CO) and the fractional distribution (FD) of .gamma.-labeled plastic microspheres (15 .+-. 5 .mu.m) injected into the left ventricle were used to calculate blood flow to organs and tissues of barbital-sedated warm-acclimated (WA) or cold-acclimated (CA) white rats at rest and then during their maximal calorigenic response to infused noradrenaline (NA). Flow to the major masses of brown adipose tissue (BAT) increased in WA rats from a mean of 0.81 ml/min (0.92% of CO) at rest to 13.5 ml/min (11.4% of CO) during calorigenesis; it increased in CA rats from 2.3 ml/min (2.6% of CO) to 57.2 ml/min (33.5% of CO). Flow to skeletal muscle increased in WA rats from 12.0 ml/min at rest to 15.1 ml/min during calorigenesis; it increased in CA rats from 9.9 ml/min to 14.5 ml/min. Flow to heart and to muscles involved in respiratory movements was 2-5 times greater during calorigenesis. Flow to most other tissues and organs increased or decreased by less than 40%. Arteriovenous differences in blood O2 ((A-V)O2) across interscapular BAT (IBAT) during rest and during calorigenesis together with measurements of blood flow established that IBAT alone accounted for 14% of the extra O2 used by CA rats during NA-induced calorigenesis. If during calorigenesis other masses of BAT have an (A-V)O2 as great as that for IBAT, the major masses of BAT together would account for 60% of the calorigenic response of the CA rat. In contrast, even if the skeletal muscle of the CA rat used all the O2 in the blood flowing through it during calorigenesis, it could not have been responsible for more than 12% of the calorigenic response. The rat, long considered to exemplify major participation of skeletal muscle in nonshivering thermogenesis (NST), becomes just 1 of a growing list of species for which there is explicit or circumstantial evidence that NST occurs principally in BAT. It becomes reasonable to propose as a general principle that BAT is the primary anatomical site of the NST that is characteristic of many small mammals: CA adults, newborns and hibernators alike.