SYMPATHETIC NEUROMUSCULAR-TRANSMISSION IN RAT TAIL ARTERY - A STUDY BASED ON ELECTROCHEMICAL, ELECTROPHYSIOLOGICAL AND MECHANICAL RECORDING

The main purpose of the present thesis was to study sympathetic neurom uscular transmission at the varicosity level, in proximal regions of t he isolated tail artery of 'adult' normotensive rat. The strategy was to compare the release and extracellular life-times of ATP and NA, as revealed by electrophysiological and electrochemical methods, respecti vely, with the contractile response to electrical field stimulation wi th a single pulse up to 20 min of stimulation at 20 Hz. The extracellu larly recorded excitatory junction current (EJC) resolved, based on ev idence in the literature, the quantal pulse-by-pulse release of ATP. T he electrochemical signal, which reflected the rise and fall in the NA concentration at the surface of the carbon fibre electrode following nerve stimulation (termed DELTA[NA]CF), monitored on-line and in real time', the changes in the NA concentration at the surface of the arter y. The total duration of the evoked EJC and DELTA[NA]CF responses indi cated that the life-time of the ATP and NA released by a single pulse, or by short trains (10 pulses) at 50 Hz, was less than 100 ms or 3 se c, respectively. The corresponding contractile responses were mediated by both ATP (or a related nucleotide) and NA (but probably not NPY) v ia postjunctional P2x-purinoceptors, alpha1- and alpha2-adrenoceptors, respectively, and occurred with long delays during which the released ATP and NA had been virtually eliminated or removed from the respecti ve receptors. The kinetics of these contractile responses are thus not determined by the duration of the presence of transmitters at the pos tjunctional receptors driving the contractions but by post-receptor me chanisms. The large differences in the time-course of these three comp onents of the neurogenic contraction and the interaction between them reveal a novel, kinetic aspect of NA-ATP co-transmission. The relative contribution of NA and ATP to die neurogenic contraction and the rela tive role of components mediated via alpha1- and alpha2-adrenoceptors varied with the length and frequency of the stimulus trains. The ATP-m ediated component of this response was always fast and normally small but could be powerful under certain conditions (e.g. in 'young' rats, or in the presence of K+-channel blocker). In addition, the results su ggest that ATP released by field stimulation restricted both component s of the NA-induced neurogenic contraction. The source(s) of the ATP e xerting this inhibitory effect and the mechanisms of the inhibition ar e not clear at present. Pharmacological block of neuronal NA reuptake enhanced the DELTA[NA]CF response to a single pulse much less strongly than the contractile response; the potentiation of the DELTA[NA]CF re sponse was relatively constant under all conditions but that of the co ntractile response inversely related to the train length and stimulati on frequency. On stimulation with longer trains (800 pulses) at 20 Hz, the ATP-mediated component of the neurogenic contraction faded rapidl y while DELTA[NA]CF and the NA-induced, alpha1- and alpha2-adrenocepto r-mediated contractile responses were relatively well maintained. All these responses 'relaxed' rapidly upon cessation of nerve stimulation. During even more prolonged stimulation (10-20 min) at 2 Hz the DELTA[ NA]CF and contractile responses were maintained, but both declined pro gressively during similar stimulation at 20 Hz. A working hypothesis i s proposed according to which (i) neuronal reuptake of released NA cur tails the contractile response to a single pulse or short trains while local overload of the reuptake transporter causes residual NA to accu mulate locally and potentiate the contractile response to high frequen cy trains, (ii) nerve activity-induced suppression of neuronal reuptak e and diffusion of released NA enables the vessel to maintain DELTA[NA ]CF and tension during long trains even when the quantal release of tr ansmitters is low, and (iii) reactivation of these clearance mechanism s enables the vessel to relax rapidly upon cessation of stimulation.