Differential effects of nifedipine and verapamil on non-adrenergic non-cholinergic relaxations at different levels of active tone: assessment of the contribution of nerve-derived hyperpolarizing factor Y. Ergu ¨n Department of Pharmacology, Faculty of Medicine, University of Cukurova, 01330, Balcali, Adana, Turkey Summary 1 The objective was to investigate a possible contribution of a nerve-derived hyperpolarizing factor to the differences between non-adrenergic non-cholinergic (NANC) nerve-mediated relaxations in different states of active tone in the rat gastric fundus. 2 NANC relaxations induced by electrical field stimulation (ES: 0.1, 0.5 and 1 Hz; 25 V; 1 ms; 10 s) in 40% contracted strips (S40) were greater when compared with those in 80% contracted strips (S80). 3 ES-induced relaxations were effectively attenuated by N x -nitro-L-arginine (L-NNA; 100 lM) in S40 and S80. Percentage reduction of the responses obtained in the presence of L-NNA in S40 group was less than that of S80. 4 In S40 group, nifedipine (0.5–1 lM) and verapamil (0.5–1 lM) inhibited the responses to 0.1 and 0.5 Hz. Nifedipine (1 lM) and verapamil (0.5 lM) caused no change in the responses to ES in S80. 5 In S40, when L-NNA (100 lM) and nifedipine or verapamil, either in 1 lM concentration, were administered together, the inhibition on the electrical relaxations were more than that of each drug alone. 6 In conclusion, NANC nerve-mediated relaxations are increased when studied in an active state of 40%, and a factor, sensitive to nifedipine seems to be responsible for this distinction. Keywords: nitric oxide, nerve-derived hyperpolarizing factor, nifedipine, gastric fundus, rat Introduction The role of nitric oxide (NO) as a neurotransmitter in gastrointestinal tract of a variety of species has been well documented (Brookes, 1993). However, most of the studies conducted to investigate the role of NO were examined in preparations con- tracted to near-maximum levels of active force. According to Selemidis & Cocks (1997), such an excessive functional antagonism might exaggerate the potency of nitric oxide synthase (NOS) inhibitors and hence might cause the hyperpolariz- ing effect to be masked, as previously determined for endothelium-derived hyperpolarizing factor in various species (Kilpatrick & Cocks, 1994; Gar- land, Plane, Kemp & Cocks, 1995; Drummond & Cocks, 1996). These investigators demonstrated non-adrenergic non-cholinergic (NANC) nerve- mediated relaxations to be effectively attenuated by NOS inhibitors in the strips contracted to approximately 70% maximum, whereas an appar- ent resistance to such agents was observed in 40% contracted preparations in the rat anococcygeus muscle (Selemidis & Cocks, 1997). It was postu- lated that an additional non-NO transmitter, termed nerve-derived hyperpolarizing factor (NDHF), takes place as the relaxant mediator when the strips are contracted below the levels of near-maximal contraction by a depolarizing stimu- lus (Selemidis & Cocks, 1997). A similar conclu- Address for correspondence: Y Ergu ¨ n, Kahramanmaras Sutcu Imam University, Faculty of Medicine, Department of Pharmacology, 41600, K. Maras, Turkiye. 105 Autonomic & Autacoid Pharmacology, 25, 105–112 Ó 2005 Blackwell Publishing Ltd Correspondence: Y. Ergu ¨n*