ANP has no postsynaptic effect on autonomic regulation of cardiac pacemaker rate in the rat

We studied whether atrial natriuretic peptide (ANP) influences sinoatrial node pacemaker activity or whether it modifies the response to activation of postsynaptic autonomic receptors. Male Sprague-Dawley rats were anesthetized with pentobarbital sodium (45 mg/kg). Their hearts were removed quickly and placed in physiological salt solution. The atria were isolated; the right intra-atrial chamber was exposed to allow intracellular recording from sinoatrial node pacemaker cells. The tissue was placed in a temperature-regulated recording chamber and superfused with warmed oxygenated physiological salt solution. With use of standard microelectrode recording techniques, action potentials were recorded from spontaneously depolarizing cells in the presence of muscarine (62.5-500 nM) or norepinephrine (0.1 and 1.0 microM). Muscarine reduced the frequency of action potentials dose dependently, whereas norepinephrine increased their frequency. The addition of ANP (0.1-100 nM) to the superfusion had no effect on the frequency of action potentials during the superfusion of physiological salt solution or in the presence of either muscarine or norepinephrine. We conclude that ANP does not act on cardiac pacemaker cells to modulate the effect of neurotransmitters.