Further evidence for the prevention by guanethidine of noradrenaline efflux from rabbit ventricular slices induced by a low external sodium concentration

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Presynaptic dual inhibitory actions of guanabenz on adrenergic transmission

Actions of guanabenz, clonidine and guanethidine on adrenergic transmission were studied in rabbit atria and ilea with intact nerves. While guanabenz 3 X 10(-6) M and clonidine 3 X 10(-7) M inhibited to a similar degree atrial positive chronotropic responses to nerve stimulation at 2 Hz, no inhibition was seen at 10 Hz. Phentolamine 3 X 10(-6) M completely antagonized the inhibition by clonidine, but a partial antagonism was seen against guanabenz. Cocaine 10(-5) M prevented the phentolamine-resistant inhibition by guanabenz. Guanethidine 10(-5) M inhibited the responses to 2 and 10 Hz. In ilea, guanabenz 10(-5) M inhibited frequency-independent relaxation responses to 2-20 Hz. Clonidine 10(-5) M inhibited preferentially the responses to lower frequencies. After the temperature of the bathing solution was kept at 4 degree C during the period of drug application, guanabenz no longer produced an inhibition of the transmission, whereas clonidine produced the usual inhibition. Thus, guanabenz appears to have both presynaptic alpha-receptor stimulating and adrenergic neuron blocking actions.

Comparative studies of (--)-, (+/-)-propranolol, atenolol, guanethidine, bretylium and tetracaine on adrenergic transmission

1 Effects of (--)-, (+/-)-, and (+)-propranolol, atenolol, guanethidine, bretylium and tetracaine were studied on relaxation responses of rabbit ileum and contractile responses of rabbit pulmonary artery and guinea-pig vas deferens to electrical nerve stimulation (2 to 50 Hz). 2 In the ileum, inhibition by tetracaine 3.3 x 10(-6) M occurred at high frequencies of stimulation, while bretylium 1.2 x 10(-4) M and guanethidine 2 x 10(-5) M inhibited responses at all frequencies, the latter producing greater inhibition at low frequencies. 3 (+/-)-Propranolol 10(-5) M produced a tetracaine-type inhibition after 1 h and a bretylium-pattern after 2 h in the ilea and pulmonary arteries and a transition from bretylium- to guanethidine-pattern in the vas deferens, while atenolol 2 x 10(-5) to 10(-4) M produced guanethidine-type inhibition in all preparations. 4 (--)-, (+/-)-, and (+)-Propranolol 3 x 10(-6) to 3.3 x 10(-5) M were equipotent in the vas deferens and ileum. However, inhibition by (--)-propranolol 3.3 x 10(-5) M persisted in the ileum, while that by the (+)-isomer was partially restored by washing. 5 (--)- or (+)-Propranolol 3.3 x 10(-5) M or atenolol 2 x 10(-5) M did not inhibit relaxation of the ileum after the bath temperature was maintained at 4 degrees C for 2 h during drug application. 6 In conclusion, propranolol and atenolol both have gradually developing guanethidine-like adrenergic neurone blocking actions.