Effect of tyramine and guanethidine on dopamine-beta-hydroxylase activity and norepinephrine concentrations in vesicular fraction of the heart and plasma of rats

Differential mechanisms of action of the trace amines octopamine, synephrine and tyramine on the porcine coronary and mesenteric artery

Trace amines such as p-tyramine, p-octopamine and p-synephrine are found in low concentrations in animals and plants. Consumption of pre-workout supplements containing these plant-derived amines has been associated with cardiovascular side effects. The aim of this study was to determine the mechanisms of action of these trace amines on porcine isolated coronary and mesenteric arteries. Noradrenaline caused contraction of mesenteric arteries and relaxation of coronary arteries. In both tissues, all three trace amines induced contractions with similar potencies and responses were unaffected by the β-adrenoceptor antagonist propranolol (1 µM), the nitric oxide synthase inhibitor L-NNA (100 µM), or the TAAR-1 antagonist, EPPTB (100 nM). However, the contractile responses of mesenteric arteries, but not coronary arteries, were significantly reduced by depletion of endogenous noradrenaline. Mesenteric responses to all three amines were abolished in the presence of prazosin (1 µM) whereas residual contractile responses remained in the coronary artery which were inhibited by a high concentration (100 µM) of EPPTB. The results suggest complex responses of the coronary artery to the trace amines, with activity at α₁-adrenoceptors and potentially TAARs other than TAAR-1. In contrast the actions of the amines on the mesenteric artery appeared to involve indirect sympathomimetic actions and direct actions on α₁-adrenoceptors.

A comparative study on the effects of diethyl ether and thiopentone sodium on plasma catecholamine levels in the rat

1. The primary aim of this investigation was to ascertain the effects of thiopentone sodium and diethyl ether as anaesthetics on plasma catecholamine levels of Sprague-Dawley and Long-Evans rats. A secondary aim was to investigate the effect of norepinephrine on susceptibility to electrically induced arrhythmia in the Langendorff perfused isolated rat heart. 2. Thiopentone sodium anaesthesia resulted in significantly lower plasma norepinephrine and epinephrine levels when compared with diethyl ether anaesthesia. 3. Long-Evans rats exhibited significantly higher epinephrine levels regarding both anaesthetics in comparison with their Sprague-Dawley counterparts. 4. If plasma catecholamines are an indicator of the level of induced stress then diethyl ether anaesthesia resulted in more stress both of a psychological and physiological origin when compared with thiopentone sodium anaesthesia. 5. Long-Evans rats seem to be more prone to the physiologically-induced component of stress than Sprague-Dawley rats. 6. In isolated hearts, norepinephrine increased susceptibility (P less than 0.002) to electrically induced arrhythmia. This occurred at a concentration corresponding to measured upper levels of total catecholamines in diethyl ether anaesthetised animals.

Differential effect of guanethidine on dopamine and norepinephrine pools in urine, heart and superior cervical ganglion in the rat

The time-related changes of dopamine (DA) and norepinephrine (NE) pools were investigated in heart, superior cervical ganglion (SCG) and urine in rats treated chronically with guanethidine (50 mg/kg i.p. five days each week). The efficiency of sympathectomy was assessed by the great loss of NE in heart and superior cervical ganglion (SCG) (-96% and -76% respectively of control values on day 18) together with the ready reduction of NE and 3-methoxy-4-hydroxyphenylglycol (MHPG) in urine. The pattern of changes was quite different for DA, which was less readily affected and at a lesser extent than NE in heart and SCG thus suggesting the presence of norepinephrine-independent DA stores. Similarly the urinary excretion of free DA, free 3,4-dihydroxyphenylacetic acid (DOPAC) and free homovanillic acid (HVA) was slightly decreased only from the 9th day, whereas urinary conjugated DA remained unaltered. These results indicate that the greatest portion of urine free and conjugated DA, free DOPAC and free HVA derives from peripheral pools located outside noradrenergic neurons. Alternatively, the time-course of DA sensitivity to guanethidine suggests that a portion of urine DA may originate from DA stored independently from NE in noradrenergic neurons.

Absence of a relationship between sympathetic neuronal activity and turnover of serum dopamine-beta-hydroxylase

The effects of pharmacological alteration of adrenergic transmission on the rate of entrance of dopamine-beta-hydroxylase (DBH) into the circulation were assessed in rats by an immunological method in which the kinetics of recovery of serum DBH activity were measured after depletion of the enzyme by treatment with anti-rat DBH antiserum. Neither alpha-receptor blockade with phenoxybenzamine nor ganglionic blockade with clorisondamine altered the rate by which DBH enters the bloodstream although both treatments markedly altered serum catecholamine levels. Prolonged treatment of newborn rats with guanethidine produced a severe peripheral sympathectomy but only a moderate decrease (30%) in serum DBH levels. In the sympathectomized rats, the rate of entrance of DBH into the circulation was significantly reduced whereas the half-life and rate of degradation of the enzyme was unchanged. These results indicate that the major portion of serum DBH does not enter the circulation by means of exocytotic release of the soluble enzyme.

Changes in plasma noradrenaline concentration as a measure of release rate

1 A method is described for repeated sampling of plasma noradrenaline (NA) in freely moving rats. Na concentration does not change during the day or after adrenalectomy. 2 Exogenous NA has a half-life of 1.5 min; drugs which block neuronal and extra-neuronal uptake lengthen this to 6.3 min. 3 Swim-stress leads to a steep rise followed by a rapid decline in plasma NA concentration. 4 This method of plasma NA sampling can serve as a measure of both steady and rapid changes in release rate over long periods of time.

Plasma adrenaline, noradrenaline and dopamine in man and different animal species

1. The catecholamines adrenaline (A), noradrenaline (NA) and dopamine (DA) were determined in plasma samples of man and various animal species using a highly sensitive radioenzymatic method.2. Basal values were determined under conditions producing virtually no physical or psychic stress in blood obtained through acutely inserted venous catheters in human volunteers, rabbits and cows, through chronic indwelling catheters in cats and rats, and by cubital venipuncture in trained dogs.3. Basal values (pg/ml.) for A, NA, and DA were respectively 64, 203 and 98 in man, 73, 609 and 276 in cats, 166, 392 and 216 in rabbits, 56, 152 and 91 in cows, 204, 376 and 173 in dogs, and 175, 509, and 84 in SPF rats. The NA concentrations were always higher than those of A and DA.4. Gentle handling of rats for 30 sec greatly increased the levels of all catecholamines, especially of A. Even more marked rises were observed during and up to 5 min after restraint stress.5. Blood from the trunk of decapitated rats contained about 20 times more A and 3-4 times more DA and NA than venous blood from catheters in the absence of handling.6. Basal values of plasma catecholamines in small animals can only be obtained through indwelling catheters and in the absence of handling. Most of the previously reported values are too high and are experimental artifacts.