A dual component analysis explains the distinctive kinetics of cAMP accumulation in brown adipocytes

The mechanism behind the distinctive non-Michaelis-Menten, bell-shaped kinetics of cAMP accumulation in brown adipocytes (which underlies the similar kinetics of UCP1 and beta(1)-adrenoreceptor gene expression) was investigated. A theoretical dual component analysis indicated that the observed dose-response curves could be constructed as the resultant of a stimulatory and an inhibitory component. Experimentally, inhibition of the alpha(1)-component of the norepinephrine response revealed the underlying existence of a much larger stimulatory beta(3)-component which displayed monophasic Michaelis-Menten kinetics. The inhibitory alpha(1)-component (which was also monophasic but had a S-fold higher EC50) was mediated via an increase in [Ca2+](i); the protein kinase C pathway was not involved. The [Ca2+](i) increase which resulted in massive inhibition of cAMP accumulation was very low: <100 nM The [Ca2+](i) signal stimulated a calmodulin-controlled phosphodiesterase, possibly PDE-1. The acquirement of this specific interaction pattern between beta- and alpha(1)-adrenergic stimulation was thus part of the differentiation program of the brown adipocytes. It was concluded that an array of synergistic or inhibitory alpha(1)/beta interactions occur in the adrenergic regulation of this cell type which is unique in its dependence upon adrenergic stimulation for cellular proliferation, differentiation, and metabolic function.