A PHARMACOLOGICAL ANALYSIS OF CALCIUM CHANNELS INVOLVED IN PHASIC ANDTONIC RESPONSES OF THE GUINEA-PIG URETER TO HIGH POTASSIUM

1 Previous studies have established a marked difference in sensitivity to organic calcium channel blockers of the phasic compared with tonic component of the contraction to potassium chloride (KCl) in the guine a-pig ureter, The mechanisms responsible for this difference have rema ined unsettled, In particular, the possible involvement of non-L-type calcium channels in contractility of the ureter has not been determine d, In this study we have re-addressed this problem and, to eliminate a ny possible contribution of sensory neuropeptides released by KCl from peripheral endings of afferent nerves, all experiments were performed in ureters pre-exposed to the sensory neurone blocking agent, capsaic in (10 mu M for 15 min). 2 Increasing concentrations of KCl (10-160 mM ) produced phasic and tonic contractions of the guinea-pig isolated ur eter: the L-type calcium channel agonist, Bay K 8644 (1 mu M), enhance d both components of the contraction to KCl. 3 Nifedipine (1 mu M) abo lished all responses to increasing concentrations of KCl after 60 min contact time; after a shorter incubation period (15 min), the phasic c ontractions to low KCl concentrations were still observed, while the t onic responses were abolished. 4 The effects of nifedipine (0.1 nM-1 m u M) on the phasic and tonic components of the response to 80 mM KCl w ere assessed after 15-120 min contact time. Nifedipine was equipotent in inhibiting the tonic response at all times tested, while a marked t ime-dependency of inhibition toward phasic responses was observed, Aft er 15 min contact time, nifedipine was 181 times more potent in inhibi ting tonic than phasic response to KCl, while after 120 min contact ti me the difference between EC(50) values was only 5.4 times. 5 Cadmium chloride (3-30 mu M) was equi-effective in inhibiting the phasic and t onic responses to KCl while nickel chloride was ineffective at 10-fold higher concentrations, omega-Conotoxin (0.1 mu M) and tetrodotoxin (0 .3 mu M) were ineffective. 6 The present findings indicate that L-type voltage-dependent calcium channels mediate both phasic and tonic comp onents of the response of the guinea-pig ureter to KCl while neither T -type nor N-type voltage-dependent calcium channels are involved. The marked time-dependency of inhibitory action of nifedipine suggests tha t L-type voltage-dependent calcium channels which are responsible for the generation of phasic contraction of the ureter are in a low affini ty state for interaction with nifedipine.