Fate of 3H-noradrenaline in canine subcutaneous adipose tissue

Relationship between the overflow of endogenous and radiolabelled noradrenaline from canine blood perfused gracilis muscle

The effects of sympathetic nerve stimulation (SNS) on the overflow of endogenous noradrenaline (NA) and on vasoconstrictor responses were studied in blood perfused canine gracilis muscle in situ. A conventional tracer technique with 3H-labelled NA (3H-NA) was used in parallel. At rest there was a net extraction of endogenous NA and adrenaline across the tissue. The SNS evoked overflow of endogenous NA was frequency-dependent and logarithmically correlated to the vasoconstrictor responses. The neuronal uptake inhibitor desipramine doubled the SNS induced overflow of endogenous NA without enhancing the vasoconstrictor responses. A further fourfold increase in NA overflow was caused by a dose of the alpha-blocker phenoxybenzamine which reduced the vasoconstrictor responses by 50-75%. Less than 10% of the spontaneous 3H efflux was recovered as unmetabolized 3H-NA, whereas virtually all 3H overflow evoked by SNS was 3H-NA. The fractional release of NA or 3H-NA per nerve impulse increased with increasing frequencies of SNS under all conditions studied. Although there was a preferential release of the newly stored radiolabelled transmitter, results concerning endogenous NA and 3H-NA overflow were qualitatively similar, also under conditions with marked changes in transmitter overflow. Endogenous NA gave a more reproducible index of transmitter overflow than did 3H-NA and, in particular, total 3H. The overflow of endogenous NA closely reflects SNS evoked neuronal release of NA in blood perfused skeletal muscle and seems more suitable than conventional radiotracer techniques for studies of NA release under in vivo conditions.

Vasodilatation and modulation of vasoconstriction in canine subcutaneous adipose tissue caused by activation of beta-adrenoceptors

The present experiments were undertaken to study the balance between vascular alpha- and beta-adrenoceptors in canine subcutaneous adipose tissue during sympathetic nerve stimulation and noradrenaline injections. Propranolol potentiated and prolonged the vasoconstrictor response to close i.a. injections of noradrenaline. The vasoconstriction induced by brief nerve stimulation (0.5 to 8 Hz) was, however, unaltered by the beta-adrenoceptor blockade. During prolonged nerve stimulation the vasoconstrictor response was well maintained at 1.5 Hz but at 4 Hz there was a gradual escape. The escape phenomenon at 4 Hz was diminished by propranolol. The beta1-selective antagonist practolol, like propranolol, potentiated and prolonged the vasoconstriction induced by noradrenaline injections and reduced the vasoconstrictor escape during prolonged nerve stimulation at 4 Hz. Furthermore, the vasodilatation induced by noradrenaline injection or nerve stimulation during alpha-adrenoceptor blockade was diminished by practolol. Practolol also blocked the lipolytic response to noradrenaline and nerve stimulation. The beta2-selective antagonist H35/25 blocked the effects of the beta2-selective agonist salbutamol but failed to alter noradrenaline as well as nerve stimulation induced vascular and lipolytic beta-adrenoceptor responses. The present results provide further support for the hypothesis that vascular beta-adrenoceptors in adipose tissue are humoral (noninnervated), preferentially activated by circulating noradrenaline. Moreover, both vascular and lipolytic beta-adrenoceptors activated by noradrenaline in adipose tissue are best classified as beta1-adrenoceptors.

Influence of adipose tissue blood flow on the lipolytic response to circulating noradrenaline at normal and reduced pH

Hypercapnic acidosis (pH 7.0) inhibits the lipolytic response of canine subcutaneous adipose tissue to i.v. infused noradrenaline (NA) by 80 per cent or more. The response to sympathetic nerve stimulation, on the other hand, is only reduced by 10-40 per cent during acidosis. The fate of intravenously infused 3H-labelled NA (0.35 ug X kg-1 X min-1 for 30 min) was not significantly altered by acidosis. The rate of disappearance of unmetabolized NA from the arterial plasma after an infusion was the same at pH 7.4 and 7.0 and the calculated increase in circulating NA during infusions was 4 ng/ml at both pH:s. I.v. infusion of Na increases adipose tissue blood flow, an effect which is attenuated by acidosis. There was a significant correlation (p less than 0.001) between adipose tissue blood flow and the lipolytic response at normal pH. Preventing the NA-induced increase in blood flow by constant flow perfusion reduced the lipolytic response at normal pH. The degree of inhibition by acidosis of the lipolytic response to i.v. NA was significantly reduced (from 79 to 56 per cent, p less than 0.05) when the adipose tissue was perfused at constant flow. These data suggest that adipose tissue blood flow is important in determining the lipolytic response to i.v. NA, probably by influencing the delivery of NA to the tissue. The marked inhibition by acidosis of lipolysis due to i.v. infused NA therefore appears to be the combined effect of a direct antilipolytic effect of acidosis and a decreased delivery of NA to the adipose tissue due to the attenuated blood flow response.

Effects of adenosine on adrenergic neurotransmission; prejunctional inhibition and postjunctional enhancement

The action of adenosine on adrenergic neuroeffector transmission was studied in the rabbit kidney in vitro and in situ, in the canine subcutaneous adipose tissue in situ and in the guinea pig vas deferens in vitro. In the kidney, adenosine (0.1-10 muM) caused a concentration-dependent increase in vascular resistance and in vasoconstrictor responses to nerve stimulation and administered noradrenaline. In the adipose tissue, adenosine also increased the vaso-constrictor responses but it decreased vascular resistance. In all three tissues studied adenosine significantly and reversibly depressed noradrenaline release evoked by nerve stimulation in a concentration-dependent manner. This effect of adenosine was not altered by phenoxybenzamine which blocked all vasoconstrictor responses and diminished the rise in vascular resistance by adenosine in the kidney. It is concluded that adenosine affects adrenergic neuroeffector transmission by two discrete mechanisms, prejunctional inhibition and postjunctional enhancement.

Release of adenosine-like material from isolated perfused dog adipose tissue following sympathetic nerve stimulation and its inhibition by adrenergic alpha-receptor blockade

Following the intraarterial infusion of 3H-adenine to isolated perfused canine subcutaneous adipose tissue, its adenine nucleotides are labelled. A continuous release of radioactivity, comprised of non-nucleotide material, was observed. The rate of this release was markedly enhanced by sympathetic nerve stimulation. The major components of the enhanced release appeared to be inosine and adenosine. Adrenergic alpha-receptor blockade (phentolamine or Hydergin) abolished the enhanced nucleoside release, while glycerol release was enhanced. The release of radioactivity was decreased during mechanical blood flow reduction and enhanced afterwards. However, the magnitude of this enhancement of release after clamp was much less than following nerve stimulation. The results suggest that adenosine or a closely related compound is released from canine subcutaneous adipose tissue by sympathetic nerve stimulation and that the release is related to adrenergic alpha-receptor stimulation. Since adenosine is a potent inhibitor of catecholamine induced lipolysis in this tissue the possibility of a regulatory role must be considered.

Role of pre- and postjunctional inhibition by prostaglandin E2 of lipolysis induced by sympathetic nerve stimulation in dog subcutaneous adipose tissue in situ

1. Canine subcutaneous adipose tissue was isolated and autoperfused in situ after labelling of the noradrenaline stores by (3)H-(-)noradrenaline.2. Prostaglandin E(2) (10-200 ng/ml) increased blood flow and glucose uptake, and caused a dose-dependent inhibition of lipolysis induced by sympathetic nerve stimulation (4 Hz). The actions of exogenous prostaglandin E(2) are therefore similar to those of prostaglandin E(1) in this tissue. There were no consistent effects of prostaglandin E(2) on the vasoconstriction or on the (3)H-noradrenaline overflow induced by nerve stimulation.3. Phenoxybenzamine (1.5-2 mg i.a.) caused a 5-fold increase in (3)H-noradrenaline overflow and a 95% reduction of the vasoconstrictor response to nerve stimulation. The lipolytic response was similar to that of the control. Prostaglandin E(2) (100-200 ng/ml) administered after phenoxybenzamine caused a 90% inhibition of lipolysis, while the vasoconstrictor response was enhanced to about 50% of control. Prostaglandin E(2) inhibited (3)H-noradrenaline overflow by about 50% but it was still larger than that of the control.4. It is suggested that exogenous prostaglandin E(2) inhibits lipolysis induced by sympathetic nerve stimulation mainly by a postjunctional action in canine subcutaneous adipose tissue.