| Summary: | The mechanisms of autonomic neuroeffector transmission were investigated in the rat anococcygeus using fluorescent Ca2+ imaging, electrophysiology with sharp microelectrodes and contraction studies. The postjunctional effects of electricallystimulated release of neurotransmitters and exogenous agonists on the smooth muscle tissue were the particular focus of this study. Possible mechanisms underlying the relationship between Ca2+ signals and electrical activity, and synchronisation of activity between the smooth muscle cells in the smooth muscle cells, are discussed. A summary ofthe key experimental results follows: Brief periods of electrical field stimulation ~EFS) of the nerves produced transient changes in intracellular cytosolic [Ca2l ([Ca li) in the smooth muscles. These took the fonn of focal transients, propagating intracellular waves and global transients that were synchronised between many contiguous smooth muscle cells. The waves and global transients were abolished by the a-adrenoceptor antagonist prazosin, though some focal events persisted. Mono- and bi-phasic depolarisations of the smooth muscle membrane potential, tenned excitatory junction potentials (EJPs), were also evoked by electrical stimulation. The a-adrenoceptor agonist, phenylephrine, at 100 nM produced asynchronous Ca2+ waves, while brief periods of EFS in the presence of 100 nM phenylephrine produced synchronised global oscillations in [Ca2+]j, which were accompanied by oscillations in membrane potential, indicating that a coupling mechanism exists between the smooth muscle cells. In intact tissue, Ca2+ waves and biphasic EJPs were not observed to be coupled at the level of the individual cell. While the rapid phase (first component) of the biphasic EJP in response to very short periods ofEFS is sensitive to caffeine and appears to be mediated by release of Ca2+ from intracellular stores, acting on Ca2+-activated cr channels, the depolarisation that is measured seems to be the sum of events in a pool of electrically-coupled smooth muscle cells. Synchronised global transients were associated with EJPs with a large first component, and were reduced in amplitude but not abolished by nifedipine, though they were not observed in the presence of a combination of caffeine and nifedipine, suggesting that L-type Ca2+ channels contribute to these events but are not essential for their generation. Guanethidine or amphetamine, which displace noradrenaline from sympathetic nerve terminals, produced Ca2+ waves and synchronised global oscillations in [Ca2li. In the presence of guanethidine, which also prevents evoked neurotransmitter release from these tenninals, EFS produced temporary inhibition of the oscillation, stopping it or slowing its rate and reducing its amplitude, though after the end of EFS the oscillation usually returned with a slightly raised amplitude. These effects were mediated largely by inhibitory nitrergic neurotransmission, because they were greatly reduced by the inhibitor ofnitric oxide synthase, L_NG-nitro-arginine-methyl-ester.
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