EFFECT OF HEMICHOLINIUM ON THE RELEASE OF AUTONOMIC MEDIATORS IN THE SINOATRIAL NODE.

For the past four years, we have used a technique of selective stimulation of intramyocardial autonomic nerve fibres in a variety of isolated heart tissue preparations. The technique is " selective " in that electrical stimuli of appropriate characteristics, when applied directly to the myocardium, are capable of exciting intramyocardial autonomic nerve fibres without exciting the myocardium per se. Selective stimulation of intramural autonomic nerve fibres is not without precedent, and has been reported for vas deferens (Birmingham Wilson, 1963) and trachea (Carlyle, 1964) as well as for heart (Ursillo, 1958; Lewartowski, 1963; Vincenzi & West, The chronotropic and inotropic responses produced by applying appropriate electrical stimuli to cardiac tissue depend on the release of acetylcholine and noradrenaline (Amory We have.used the term " electro-release" to describe this general phenomenon in order to differentiate it from what might be termed " chemo-release" Shideman, It should be noted that the stimuli used in this study, although applied directly to the sinoatrial node, were subthreshold for the specialized cardiac tissue of the sinoatrial node, but were suprathreshold for intranodal autonomic nerve fibres. spontaneous beat interval were used to characterize each chronotropic response. These were: the steady state interval just before stimulus application; the maximal interval after the beginning of nerve stimulation; and the minimal interval during or following nerve stimulation. For a given chronotropic response, the difference between the steady state interval and the maximal interval is directly related to the cholinergic electro-release response. Likewise, the difference between the steady state interval and the minimal interval is directly related to the adrenergic electro-release response.