Effect of hypoxia on nerve-stimulation-induced release of noradrenaline from the rabbit heart

The present study addressed the hypothesis that cardiac production of adenosine (ADO) and/or prostacyclin (PGI2) during hypoxia is augmented to a level sufficient to affect nerve-stimulation-induced release of noradrenaline (NA). Innervated rabbit hearts were perfused at high (95% O2) or low (8% O2) oxygen pressure. The effluxes of NA and purines from the heart were determined by HPLC and that of the PGI2 metabolite by radioimmunoassay. Five minutes of hypoxia elevated effluent purines (sum of ADO, inosine, and hypoxanthine) from 1.1 microM to 6.2 microM, but did not affect the outflow of NA. The ADO receptor antagonists THEO (100-200 microM) and 8PSOT (100 microM) given during hypoxia increased the evoked outflow of NA by 77% (P less than 0.01) and 37% (P less than 0.05), respectively. Indomethacin (30 microM, a prostaglandin synthesis inhibitor) reduced the efflux of PGI2 metabolite by 93% but did not per se affect NA outflow during simultaneous administration of THEO, either under normoxia or hypoxia. It is concluded that ADO, but not PGI2, plays a role in reducing transmitter release during hypoxia. In addition, hypoxia leads to an enhancement of transmitter release, probably unrelated to ADO or purines. The lack of effect of hypoxia alone on evoked outflow of transmitter seems to be the result of a combination of these two processes.     

Ischaemia-induced release of noradrenaline and creatine kinase in the isolated, working rat heart

In order to examine an early ischaemia-induced local release of myocardial noradrenaline (NA), the left coronary artery of the isolated working rat heart was ligated for periods varying between 7.5 min and 30 min, followed by reperfusion for 5 min. As perfusion substrate, glucose, lactate or both were used. In part of the experiments, hearts were pre-labelled with [3H]NA. Already after 7.5 min of ischaemia, an increased efflux of endogenous NA was observed in the perfusate at reperfusion, concomitant with a decrease in tissue NA content. This effect was most pronounced with lactate as substrate. Qualitatively similar effects were seen on [3H]NA efflux from labelled hearts. The combination of glucose and lactate as substrate markedly reduced (compared with lactate alone) the efflux of NA, whereas no such reduction was observed on the efflux of creatine kinase (CK). It is concluded that ischaemia is associated with an early local release of NA. Furthermore, ischaemia may induce different effects on the metabolic processes of the myocyte as compared with the adrenergic nerve ending.     

Interaction between direct sympathetic and vagus nerve stimulation on heart rate in the isolated rabbit heart

The interaction between the effects of vagus nerve stimulation (VS) and sympathetic stimulation (SS) on intrinsic heart rate was studied in the novel innervated isolated rabbit heart preparation. The effects of background VS, at different frequencies--2 Hz (low), 5 Hz (medium), 7 Hz (high)--on the chronotropic effects of different frequencies of SS--2 Hz (low), 5 Hz (medium), 10 Hz (high)--were studied. The experiments were repeated in the reverse direction studying the effects of different levels of background SS on the chronotropic effects of different levels of VS. Background VS reduced the overall positive chronotropic effect of SS at steady state in a frequency dependent manner and the rate of increase in heart rate during low and medium SS (but not high SS) was slowed in the presence of background VS. These results suggest that pre- and postjunctional mechanisms may be involved in the sympatho-vagal interaction on heart rate. On the other hand, the chronotropic effect of VS was enhanced in the presence of background SS. Vagal stimulation appears to play a dominant role over sympathetic stimulation in chronotropic effects on the isolated heart. The innervated isolated heart preparation is a valuable model to study the complex mechanisms underlying the interaction between sympathetic and parasympathetic stimulation on cardiac function.     

Estrogen inhibition of noradrenaline release in the rabbit oviduct

In the present study, we investigated the influence of estrogen on 3H-noradrenaline (3H-NA) release induced in the oviductal isthmus by electrical stimulation, potassium and calcium. The fractional release of 3H-NA was measured in oviducts isolated from ovariectomized rabbits and from ovariectomized rabbits treated with estradiol cypionate, 70 micrograms/kg im 72 h before an experiment. Electrical field stimulation of the intramural nerves induced muscle contraction and augmented the release of labelled NA from the muscle. The 3H-NA release was reduced after estrogen treatment when reuptake of NA into the nerve terminals was blocked by desipramine, 10(-6) M. Estrogen also reduced the 3H-NA release evoked by exposure of the oviducts to 121 mM KC1 in the presence of calcium (2.5 mM) and in a high potassium, calcium-free medium upon the addition of 2.5 mM calcium. In the presence of desipramine a small fraction of 3H-NA was released in high potassium, calcium-free medium. This release was unaffected by estrogen. These results suggest that estrogen reduces the release of NA from the adrenergic nerves within the oviduct and that this action is exerted primarily on the calcium-dependent release. It therefore might be due to a reduction in the entry of calcium into the nerve terminal.     

Role of the coronary endothelium in the regulation of sympathetic transmitter release in isolated rabbit hearts

The roles of endothelium-derived relaxing factor (EDRF) and endothelin in the regulation of vascular tone are intensely studied at present. Since factors which directly affect vascular tone also frequently modulate sympathetic transmitter release, we found it of interest to study whether EDRF or endothelin displays such modulatory activity as well. Isolated rabbit hearts were perfused according to Langendorff, and the release of transmitter induced by sympathetic nerve stimulation was estimated by analysis of the effluent content of noradrenaline (NA) with liquid chromatography. The activity of EDRF spontaneously formed in the heart was counteracted by addition of haemoglobin (Hb, 2.4-15 g l-1) or facilitated by addition of superoxide dismutase (SOD, 14-140 U ml-1), to the perfusion solution. In other experiments authentic endothelin (0.1-10 nm) was given to the heart. Nerve stimulation (5 Hz for 30 s) elicited a release of NA into the cardiac effluent amounting to 319 +/- 28 pmol (n = 53). Hb lowered the coronary flow to 68 +/- 9% (P less than 0.01) and impaired the outflow of NA to 62 +/- 9% of control (P less than 0.01). SOD facilitated coronary flow by 11 +/- 4% (P less than 0.005), and augmented the outflow of NA by 15 +/- 6% (P less than 0.05). Endothelin dose-dependently inhibited the coronary flow, with an IC50 of about 1 nM, and in parallel decreased the efflux of NA. Mechanical obstruction of the coronary flow induced an attenuation of the efflux of NA that was quantitatively similar to the flow reduction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estrogen inhibition of noradrenaline release in the rabbit oviduct1

In the present study, we investigated the influence of estrogen on ³H-noradrenaline (³H-NA) release induced in the oviductal isthmus by electrical stimulation, potassium and calcium. The fractional release of ³H-NA was measured in oviducts isolated from ovariectomized rabbits and from ovariectomized rabbits treated with estradiol cypionate, 70 μg/kg im 72 h before an experiment. Electrical field stimulation of the intramural nerves induced muscle contraction and augmented the release of labelled NA from the muscle. The ³H-NA release was reduced after estrogen treatment when reuptake of NA into the nerve terminals was blocked by desipramine, 10^-6 M. Estrogen also reduced the ³H-NA release evoked by exposure of the oviducts to 121 mM KCI in the presence of calcium (2.5 mM) and in a high potassium, calcium-free medium upon the addition of 2.5 mM calcium. In the presence of desipramine a small fraction of ³H-NA was released in high potassium, calcium-free medium. This release was unaffected by estrogen. These results suggest that estrogen reduces the release of NA from the adrenergic nerves within the oviduct and that this action is exerted primarily on the calcium-dependent release. It therefore might be due to a reduction in the entry of calcium into the nerve terminal.     

On-line electrochemical monitoring of the local noradrenaline release evoked by electrical stimulation of the sympathetic nerves in isolated rat tail artery

A treated carbon fibre electrode was used to measure by differential normal pulse voltammetry or differential pulse amperometry the release of noradrenaline from the sympathetic nerve terminals innervating the smooth muscle in rat tail artery. On calibration in vitro with exogenous noradrenaline in phosphate-buffered saline solution the electrode recorded an oxidation current at +0.1 V, the oxidation potential of noradrenaline. This signal was proportional to the noradrenaline concentration in the solution. When the electrode was apposed to the wall of the artery there was no oxidation current at +0.1 V under resting conditions, but electrical nerve stimulation for 1-100 s at 1-10 Hz induced a current with a peak at this potential. This signal was suppressed by tetrodotoxin, guanethidine or cadmium, or by omission of calcium; it was strongly enhanced by tetraethylammonium and potentiated by the noradrenaline uptake blockers desipramine or cocaine. The results indicate that the carbon fibre electrode method described here may be used to monitor on-line the nerve stimulation-induced increase in the local noradrenaline concentration at the surface of the muscle layer in a blood vessel such as the rat tail artery.