Cyclic nucleotide regulation of neurotransmission in guinea pig mesenteric artery

Beta-adrenoceptor mediated facilitation of noradrenaline and adenosine 5'-triphosphate release from sympathetic nerves supplying the rat tail artery

1. The effects of prejunctional beta-adrenoceptor activation on electrically evoked noradrenaline (NA) and adenosine 5'-triphosphate (ATP) were studied by use of continuous amperometry and conventional intracellular recording techniques. Excitatory junction potentials (e.j.ps) were used as a measure of ATP release, and NA-induced slow depolarizations and oxidation currents as measures of NA release, from postganglionic sympathetic nerves innervating the rat tail artery in vitro. 2. Isoprenaline (0.1 microM) increased the amplitude of e.j.ps, slow depolarizations and oxidation currents evoked by short trains of stimuli at 1 to 4 Hz. The facilitatory effect of isoprenaline on e.j.ps and oxidation currents was most pronounced on responses evoked by the first stimulus in a train. 3. Isoprenaline (0.1 microM) did not detectably alter the amplitude-frequency distribution of spontaneous e.j.ps. 4. The facilitatory effect of isoprenaline on e.j.ps, slow depolarizations and oxidation currents was abolished by the beta-adrenoceptor antagonist, propranolol (0.1 microM). Propranolol alone had no effect on e.j.ps, slow depolarizations or oxidation currents. 5. Thus, activation of prejunctional beta-adrenoceptors increases the release of both NA and ATP from postganglionic sympathetic nerves. The findings are consistent with the hypothesis that NA and ATP are released from the same population of nerve terminals and presumably from the same vesicles.

The role of cyclic AMP in non-adrenergic non-cholinergic contraction in guinea-pig bronchi

1. We investigated the role of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in non-adrenergic non-cholinergic (NANC) contraction in guinea-pig bronchial strips. 2. Forskolin (3 nM to 1 microM) reduced NANC contraction induced by electrical field stimulation (EFS) in a concentration-dependent fashion (-log EC50 was 7.22 +/- 0.12 M and maximum inhibition was 100 +/- 0.01%). However, forskolin (less than 1 microM) did not alter the contraction induced by substance P (SP, 1 microM). 3. Dibutyryl cyclic AMP (1 mM) also reduced NANC contractions induced by EFS (100 +/- 0.01%) without significant effect on SP (1 microM)-induced contractions. In contrast, dibutyryl cyclic GMP (1 mM) was without effect against either NANC or SP-induced contractions. 4. Both the beta 2-adrenoceptor agonist, procaterol (0.1 nM to 3 nM) and theophylline (100 nM to 1 mM) concentration-dependently reduced EFS-induced NANC contractions without significant effect on SP (1 microM)-induced contractions. 5. In contrast to forskolin, procaterol and theophylline, both sodium nitroprusside and cromakalim inhibited the EFS-induced contractions only at those concentrations that similarly reduced the contractions induced by SP (1 microM). 6. These results suggest that cyclic AMP may mediate pre-junctional inhibition of NANC contractions in guinea-pig bronchi.

Cholera toxin and Gs protein modulation of synaptic transmission in guinea pig mesenteric artery

Cholera toxin (CTX) was used to test whether the presynaptic beta-adrenoceptors of guinea-pig mesenteric artery are coupled via stimulatory GTP-binding proteins. The vascular smooth muscle cells were electrically quiescent unless stimulated and had a mean resting potential of -68.7 +/- 2.8 mV (n = 16) and input resistance of 12.1 +/- 0.5 M omega (n = 4). Perivascular nerve stimulation with brief square pulses evoked excitatory junction potentials (EJPs) in the muscle cells. Isoproterenol (0.1 microM) enhanced the EJP amplitude without modifying the passive membrane properties of the muscle cells. The beta-blocker, propranolol (0.5 microM), prevented the effects of isoproterenol on EJP amplitude. The permeant analogue of cyclic AMP, 8-bromocAMP, also potentiated EJP amplitude. EJP amplitude was markedly enhanced by treatment of the isolated blood vessels with CTX (10 micrograms/ml for 1 h). The muscle cells became hyperpolarized (-74.6 +/- 2.1 mV, n = 5), and their input resistances were significantly reduced (8.2 +/- 0.5 M omega, n = 4). These effects of CTX persisted after washout. Addition of GM1 ganglioside (5 micrograms/ml) prevented the CTX effects. The CTX enhancement of EJP amplitude was not prevented by application of depolarizing current (ca. 0.5 nA) the muscle cells (to counter the hyperpolarization). These results suggest that CTX increases the neurotransmitter release from the nerve terminals; the hyperpolarization may be due to an increase in K+ conductance. These effects of CTX may be mainly due to elevation of cAMP in the nerve terminal and in the muscle cell.