Abstract
A pig model in vivo was used to confirm the unique specificity for stimulation of porcine adipose tissue lipolysis by norepinephrine analogues in vitro. Plasma free fatty acid and blood glycerol concentrations were monitored as probable indicators of adipose tissue lipolysis. Plasma glucose and lactate concentrations, blood pressure, and heart rate were monitored also. Norepinephrine analogues were infused intravenously. Several compounds, classified as either beta 1- or beta 2-adrenergic agonists, that stimulated lipolysis in vitro also increased plasma free fatty acid and blood glycerol concentrations in vivo. Tazolol (beta 1) and quinterenol (beta 2) did not stimulate lipolysis in vitro and likewise did not elevate plasma free fatty acid or blood glycerol concentrations in vivo. Clenbuterol and zinterol did not stimulate lipolysis in vitro but elevated plasma free fatty acid concentrations in vivo, implying indirect effects. Isoproterenol stimulation of plasma free fatty acid and blood glycerol concentrations in vivo was antagonized by propranolol, implying the beta-adrenergic nature of the receptors. Infusion of purported beta 1- and beta 2-adrenergic antagonists suggested control of lipolysis in vivo predominantly by beta 1-adrenergic receptors; however, because the results in vitro do not indicate this specificity, differential pharmacodynamics of the antagonists are suggested rather than designation of receptor subtypes. There was no evidence for alpha-adrenergic mediated inhibition of adipose tissue lipolysis in vivo, confirming observations in vitro.
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