Studies on the cholinergic receptors in sinoauricular node and adjacent auricular tissues of cat

Cholinergic receptors in pacemaker and contractile tissue of heart were studied in an isolated sinoauricular (s.a.) node preparation of cat containing crista terminalis. Acetylcholine (ACh), 10(-9) - 10(-5), caused concentration dependent negative inotropic and negative chronotropic effects, the former being more prominent than the latter. Atropine (10(-10) - 10(-4)M) prevented these inhibitory effects of ACh in a competitive manner. No cardioexcitatory effect of ACh was seen in these preparations treated with up to 10(-4)M atropine. It is concluded that the pacemaker cells of s.a. node and contractile atrial cells in cat contain inhibitory muscarinic receptors but no ACh sensitive excitatory nicotinic receptors. The contractile cells of atrium are more sensitive than the pacemaker cells of s.a. node of cat to the inhibitory effects of ACh.