Electrical stimulation of the ventromedial hypothalamic area in rats caused a significant but transient increase in interscapular brown adipose tissue temperature. This response was markedly reduced by cimetidine, a histamine H2-receptor antagonist, but not by pyrilamine, an H1-receptor antagonist. Histamine is present in substantial amounts within mast cells in brown adipose tissue as injections of compound 48/80, which cause degranulation of connective tissue mast cells, reduced the tissue histamine content by > 85%. In contrast, histamine content in brown adipose tissue was not affected by loss of sympathetic neural input (with 6-hydroxydopamine) or sensory neural input (with capsaicin). Neither cimetidine nor histamine had any effect on basal and norepinephrine-stimulated rates of O2 consumption by isolated brown adipocytes. These results indicate that histamine released from mast cells acting on H2-receptors may play an important but indirect role in the thermogenic response of brown adipose tissue to stimulation of the ventromedial hypothalamic area.
Changes in numbers and heparin content of peritoneal fluid mast cells of growing rats measured by flow cytofluorometry.