Exposure to cold induces complex physiological response in interscapular brown adipose tissue (IBAT), which is the main site for thermogenesis. Recent findings showed that nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) displays beneficial properties on numerous physiological processes in IBAT. However, high concentration of NO induces apoptosis in several cell types. In this regard, NO may be implicated in induction of apoptosis in IBAT. Hence, the goal of present study was to examine possible relationship between NO, induction of apoptosis and processes involved in IBAT hyperplasia occurring during new homeostasis reaching, when animals were exposed to cold.

The male Mill Hill hybrid hooded rats were divided into three main groups. One group was receiving N^ω−nitro-L-arginine methyl ester (L-NAME HCl, 0.01%), an inhibitor of nitric oxide synthases (NOSs) and another L-arginine HCl (2.25%), a substrate for NOSs in drinking water for 45 days. The third group served as a control. Animals of all three groups additionally were divided in two subgroups: one – housed at 22 ±1°C and another – in cold room at 5 ±1°C.

We show here that cold exposure of rats markedly decreased the iNOS immunopositivity i.e. production of NO, and caused a rapid decrease in the apoptosis in tissue. On cold, higher iNOS immunopositivity was detected only in L-Arg treated group. Besides, in L-Arg treated group of rats acclimated to low temperature level of uncoupling protein 1 (UCP1) was higher, while, in L-NAME treated group level of UCP1 was significantly decreased in comparison to control group acclimated to low temperature. This data indicate that NO additionally induced UCP1. Similarly, L-arginine treatment, namely NO, additionally improved cold-induced tissue mass increase in contrast to L-NAME treatment, which decrease tissue mass. We also show, on room temperature, that marked increase in the iNOS immunopositivity and rate of apoptosis were detected in both treated groups of rats compared to appropriate control. These data indicates that NO produced in high concentration induced apoptosis in both treated groups of rats acclimated to room temperature.

Our results indicates that NO plays regulatory roles in IBAT hyperplasia (induction of UCP1 and increase of the tissue mass). In contrast, NO produced by high expressed iNOS induce apoptosis and can be cytotoxic.