title: The determination of presynaptic pA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition.
abstract: 1 Rat isolated perfused hearts with the right sympathetic nerves attached were loaded with [3H]-(-)-noradrenaline. The nerves were stimulated with up to 40 trains of 10 pulses every min at 1 Hz, and the evoked increases of [3H-]noradrenaline overflow into the perfusate, of right atrial tension development and ventricular beating frequency were measured. 2 Oxymetazoline inhibited the evoked transmitter overflow (IC50: 10 nM) and decreased the postsynaptic responses in a concentration-dependent manner. It behaved as a full against in abolishing the evoked transmitter overflow. 3 Yohimbine up to 1 microM neither enhanced the evoked [3H]-noradrenaline overflow nor the postsynaptic parameters. Phentolamine (1 microM) caused a transient, minor (less than 30%) increase in [3H]-noradrenaline overflow. 4 Yohimbine (0.03-1.0 microM) and phentolamine (0.1-5.0 microM) shifted to the right the concentration-response curve of oxymetazoline for the inhibition of [3H]-noradrenaline overflow in response to nerve stimulation without depressing the maxima. The pA2 values were 7.82 and 7.52, respectively. 5 Yohimbine (0.1 microM) also antagonized the decrease induced by oxymetazoline in the postsynaptic responses to nerve stimulation. 6 The results confirm the existence of presynaptic inhibitory alpha 2-adrenoceptors at the adrenergic nerve fibres of the rat heart in vitro. Under the stimulation and perfusion conditions selected, the released endogenous transmitter apparently does not activate a negative feedback mechanism, thus permitting the determination of pA2 values.
mesh_terms: D000818:Animals; D004558:Electric Stimulation; D006321:Heart / Q000187:drug effects* / Q000294:innervation; D008297:Male; D009206:Myocardium / Q000378:metabolism; D010109:Oxymetazoline / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000502:physiology; D013569:Synapses; D015016:Yohimbine / Q000494:pharmacology*
chemical_list: D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D010109:Oxymetazoline; D010646:Phentolamine