Beta2-adrenoceptor-mediated prejunctional facilitation and postjunctional inhibition of sympathetic neuroeffector transmission in the guinea pig vas deferens.

This study examines the role of prejunctional and postjunctional beta-adrenoceptors in the modulation of sympathetic cotransmission in the guinea pig vas deferens. The prejunctional involvement of beta-adrenoceptors was evaluated by testing the effects of several agonists and antagonists on the nerve stimulation-evoked overflow of ATP and norepinephrine (NE) from the "in vitro" vas deferens. The nonsubtype-selective beta-adrenoceptor agonist isoproterenol and the beta2-subtype-selective agonist clenbuterol increased, to a similar degree, the overflow of ATP and NE, while the beta1-subtype-selective agonist xamoterol and the beta3-subtype-selective agonist BRL 37 344 had no effect. Pretreatment with ICI 118, 551, a beta2-subtype-selective antagonist, abolished the facilitation of cotransmitter release by isoproterenol and clenbuterol, while the beta1-subtype-selective antagonist atenolol had no effect. Activation of beta-adrenoceptors by either isoproterenol or clenbuterol, but not by xamoterol and BRL 37 344, reduced the amplitude of contractions evoked by exogenously applied ATP. Pretreatment with propranolol or ICI 118, 551, but not atenolol, prevented these inhibitory effects. Isoproterenol in lower concentrations produced dose-dependent reduction of the purinergic but not the adrenergic phase of nerve stimulation-induced contraction of the guinea pig vas deferens. When applied in concentrations greater than 1 microM, isoproterenol, but not clenbuterol, actually produced a concentration-dependent facilitation of contractions evoked by both nerve stimulation and exogenously applied ATP. Antagonists of alpha-adrenoceptors blocked these facilitatory effects. Together, these results demonstrate that beta2-adrenoceptors can influence sympathetic neuroeffector transmission both prejunctionally, where they facilitate equally well the release of sympathetic cotransmitters and postjunctionally, where they inhibit smooth muscle contractions evoked by ATP.

Differential cotransmission in sympathetic nerves: role of frequency of stimulation and prejunctional autoreceptors.

Recent reports have suggested that sympathetic nerves may store separately and release independently the cotransmitters ATP and norepinephrine (NE). It is conceivable therefore that the quantity of each neurotransmitter that is released from the nerves is not fixed but rather may vary, possibly with the frequency of stimulation. To test this hypothesis we studied the concomitant release at various frequencies and cooperative postjunctional actions of ATP and NE during the first 10 s of electrical field stimulation of the guinea pig vas deferens. We found that at lower frequencies (8 Hz), prejunctional inhibition of the release of NE, which occurs via alpha2-adrenoceptors, modulates the ultimate composition of the cocktail of cotransmitters by limiting the amount of NE that is coreleased with ATP. As the frequency of stimulation increases (above 8 Hz), the autoinhibition of the release of NE is overridden and the amount of NE relative to ATP increases. The smooth muscle of the guinea pig vas deferens reacts to changes in composition of the sympathetic neurochemical messages by increasing the amplitude of its contractions due to the enhancement by NE of the contractile responses triggered by ATP. This evidence suggests that the prejunctional alpha2-adrenoceptor may function as a sensor that "reads" the frequency of action potentials produced during a burst of neuronal activity and converts that information into discrete neurochemical messages with varying proportions of cotransmitters. The mechanism for decoding the informational content of these messages is based on the cooperative postjunctional interactions of the participating cotransmitters.

Differences between the regulation of noradrenaline and ATP release.

1. We have studied the effects of adrenergic receptor agonists and antagonists and various calcium channel antagonists on the overflow of adenine nucleotides (ATP, ADP, AMP), adenosine (ADO) and noradrenaline (NA) from superfused guinea-pig vasa deferentia evoked by electrical field stimulation (EFS). 2. Samples of superfusate were taken at 10 s intervals for analysis of purines (HPLC with fluorescence detection) and of NA (HPLC with electrochemical detection). During 1 min of EFS the overflow of ATP peaked at about 20 s and then abruptly decreased even though stimulation continued. The overflow of NA reached a peak at about 40 s and remained at a constant level for the duration of the stimulation. 3. Pretreatment with the alpha 2-receptor antagonists idazoxan and yohimbine produced a substantial increase in the overflow of NA and a lesser increase in the overflow of ATP, indicating that endogenously released NA has a greater influence on its own release than on that of ATP. Interestingly, certain alpha 2-agonists. e.g. xylazine and clonidine, produce a greater reduction in ATP release than NA. Together the results suggest that the release of ATP and NA may be regulated by different subsets of prejunctional alpha 2-receptors. 4. The N-type calcium channel antagonist omega-conotoxin reduced the EFS-evoked release of NA to a greater extent than ATP while the P-type calcium channel antagonist omega-agatoxin did the reverse. These results indicate that NA release may be more dependent on calcium influx through N-type channels whereas ATP release is coupled to calcium entry through P-type channels. 5. These differences in the pharmacological regulation of ATP and NA release lend credence to the idea that these two co-transmitters originate from different release sites in adrenergic nerves.