Prejunctional alpha-adrenoceptor mediated prostaglandin releasing effect of clonidine in the isolated perfused rabbit kidney.

The effects of clonidine (CLO) were assessed in the isolated perfused rabbit kidney on responses to periarterial stimulation (PS) and intraarterially injected noradrenaline (NA). Changes in perfusion pressure (PP) and responses of venous outflow-superfused assay organs to PS and NA were considered as parameters. Graded increases in PP and contractions of venous outflow-superfused rabbit aortic strips were elicited by PS at frequencies of 5-10 Hz and intraarterially injected NA at concentrations of 10(-8) - 10(-7) M. CLO added to the perfusion medium caused concentration-dependent reductions of PP and of aorta contractions to PS. The inhibitory effect of CLO was reversed by acetylsalicylic acid (ASA). The responses to venous outflow-superfused rat stomach fundus strip to PS and exogenous NA were biphasic. The first relaxation phase to PS was reduced but the second contractile phase was enhanced by CLO. Responses to exogenous NA in PP and venous outflow-superfused organs were not affected by CLO. The relaxation of submaximally contracted and kidney venous effluent-superfused cat tracheal muscle to PS was reduced whereas the responses to NA was unaffected by CLO. The inhibitory effect of CLO on PS in the cat tracheal muscle was prevented by yohimbine (YOH). YOH, however, caused a further decrease in PP in response to both PS and exogenous NA. From these results it was concluded that CLO may act on presynaptic alpha-adrenoceptors in the kidney resulting in a reduction on the release of NA and enhancement of the synthesis of prostaglandins (PGs).