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

TLDR

Although skeletal muscle was long thought to drive nonshivering thermogenesis in rats, evidence now indicates that brown adipose tissue is the principal site of heat production. The study measured organ blood flow in warm‑ and cold‑acclimated rats by injecting γ‑labeled microspheres into the left ventricle and calculating cardiac output and fractional distribution during rest and during maximal noradrenaline‑induced calorigenesis. Noradrenaline markedly increased blood flow to brown adipose tissue—by more than tenfold in warm‑acclimated and by over twenty‑fold in cold‑acclimated rats—accounting for roughly 60 % of the calorigenic response, while skeletal muscle and other tissues showed only modest changes.

Abstract

Cardiac output (CO) and the fractional distribution (FD) of γ-labeled plastic microspheres (15 ± 5 μm) 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 two to five times greater during calorigenesis. Flow to most other tissues and organs increased or decreased by less than 40%.Arteriovenous differences in blood oxygen [Formula: see text] 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 O 2 used by CA rats during NA-induced calorigenesis. If during calorigenesis other masses of BAT have an [Formula: see text] 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 O 2 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), now becomes just one of a growing list of species for which there is explicit or circumstantial evidence that NST occurs principally in BAT. It thus 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.