Developmental changes in sympathetic contraction of the circular muscle layer in the guinea-pig vas deferens

Contractile responses of the circular muscle of the isolated vas deferens to electrical stimulation (10-80 Hz) and to noradrenaline significantly decreased with increasing age in 3-week-, 10-week- and 18-month-old guinea pigs, observed by the cannula insertion method. There were no significant differences in the contractile responses induced by alpha,beta-methylene ATP or BaCl2 between 3 and 10 weeks old, but the responses to alpha,beta-methylene ATP or BaCl2 decreased in 18-month-old guinea pigs. The contractile response to electrical stimulation was monophasic in 3-week-old guinea pigs, a small portion of which remained after the treatment with prazosin. Desensitisation of P2X-purinoceptors with alpha,beta-methylene ATP significantly inhibited the contractile responses to stimulation with relatively low frequencies, and the combination of both prazosin and alpha,beta-methylene ATP abolished the stimulation-induced contractions. In 10-week- and 18-month-old guinea pigs electrical stimulation evoked a transient contraction followed by a second contraction at the offset of the stimulation (the after-response). The after-responses were blocked by prazosin. These results show that the dominant component of sympathetic cotransmission is noradrenaline; a purinergic component also exists in the sympathetic contraction in the circular muscle of the vas deferens in young guinea pigs, but is virtually absent in the later stages of development. The sympathetic contractions of the circular muscles significantly decrease with increasing age and this appears to be due to changes in postjunctional, rather than prejunctional, mechanisms.