1. The chronotropic effects of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) were investigated using injections (50 micrograms in 1 ml of Tyrode solution as bolus over 1 min) directly into the sinus node artery of 21 anaesthetized and vagotomized dogs which had been pretreated with a beta-adrenoceptor antagonist. The injections were also repeated following: (a) alpha-adrenoceptor antagonism (prazosin) and muscarinic receptor antagonism (atropine); (b) inhibition of prostaglandin synthesis (indomethacin); (c) angiotensin II AT1 receptor antagonism (losartan); (d) histamine H1 (mepyramine) and H2 (cimetidine) receptor antagonism. 2. The results obtained indicate that ANP had no significant effect on the basal sinus rate, whereas CNP produced a slight but significant increase of 12 +/- 2 beats min-1. The effect of CNP was long-lasting (return to pre-injection levels after maximum effect in 17 +/- 3 min) and was not influenced by the various antagonists mentioned above. 3. During in vitro experiments on spontaneously beating right atria isolated from 6 dogs, the injection of CNP (50 micrograms in 1 ml of Tyrode solution) into the sinus node artery produced an increase in atrial rate of 14 +/- 1 beats min-1. 4. The results of this work indicate that CNP exerts a significant and prolonged positive chronotropic effect both in vivo and in vitro. Other studies are required to elucidate the mechanism of action of CNP on the heart conduction system, to ascertain the presence of natriuretic peptide receptor B in the region of the sinoatrial node and to determine the role of CNP in the control of heart rate.
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