THE FADE OF THE PURINERGIC NEUROGENIC CONTRACTION OF THE GUINEA-PIG VAS-DEFERENS - ANALYSIS OF POSSIBLE MECHANISMS

The purinergic response of the guinea-pig vas deferens to long trains of pulses at high frequency consists of an initial twitch followed by a much lower plateau. Mechanical, neurochemical and electrophysiologic al techniques were used to examine the reason for the fade. Mechanical measurements. In tissues stimulated by trains of 180 pulses/10 Hz and treated with prazosin to suppress the noradrenergic contraction compo nent, the response to alpha,beta-methylene ATP and to exogenous ATP wa s as high during the secondary plateau of the purinergic neurogenic co ntraction as it was outside electrical stimulation periods; the respon se to 50 pulses/100 Hz was also unchanged during the low plateau. The plateau was not increased by reactive blue 2,8-(p-sulphophenyl)theophy lline, propranolol or capsaicin. Neurochemical measurements. In tissue s preincubated with [H-3]-noradrenaline, electrical stimulation elicit ed an overflow of tritium and of ATP. In the absence of drugs as well as in the presence of prazosin and suramin to suppress contractions, t he overflow of tritium per pulse decreased slightly in the course of t rains of 90 pulses/10 Hz; the overflow of ATP per pulse decreased to a greater extent on average, but the decrease was not statistically sig nificant. In the presence of prazosin and nifedipine, also to suppress contractions, the overflow of tritium per pulse again decreased sligh tly in the course of trains of 105 pulses/10 Hz, but the overflow of A TP per pulse if anything tended to increase. Electrophysiological meas urements. Extracellular recording in the presence of prazosin showed t hat electrical stimulation by 180 pulses/10 Hz elicited excitatory jun ction currents (EJCs) which facilitated and summated to reach threshol d for the initiation of action potentials in the smooth muscle cells. In most tissues, smooth muscle action potentials ceased after a few se conds although EJCs continued. Intracellular recording in the presence of prazosin and nifedipine showed that excitatory junction potentials (EJPs) elicited by 180 pulses/10 Hz facilitated and summated to a pla teau after about 20 stimuli. The EJPs continued unchanged, and the pla teau depolarization was maintained, throughout the train. It is conclu ded that the fade of the purinergic neurogenic contraction is not due to P-2X-purinoceptor desensitization. It also is not due to a secondar y relaxation mediated by P-2Y- or P-1-purinoceptors, beta-adrenoceptor s or a compound originating from primary afferent axons. Moreover, a f ade of the release of ATP in the course of the pulse train is not resp onsible for the contraction fade. Rather, the reason is a failure of t he process by which the smooth muscle cell depolarization triggers act ion potentials. Inactivation of L-type Ca2+ channels that are under th e control of released ATP may be the underlying mechanism.