Co-release of endogenous ATP and noradrenaline from guinea-pig mesenteric veins exceeds co-release from mesenteric arteries.

1. The present study was designed to compare the overflow of sympathetic neurotransmitters of guinea-pig inferior mesenteric artery and mesenteric vein evoked by electrical field stimulation (EFS) with special emphasis on the simultaneous release of ATP and noradrenaline (NA). The stimulation-evoked overflow of ADP, AMP and adenosine was also evaluated. 2. Endothelium-denuded segments of inferior mesenteric arteries or veins were superfused in a small volume (200 microL)-chamber for EFS and subsequent detection of NA (HPLC- electrochemical detection) and adenine nucleotides and adenosine (HPLC-fluorescence detection) in samples of the superfusate. 3. Both arteries and veins responded to EFS (15 V, 4-16 Hz, 0.3 msec for 60 s) with overflow of ATP and NA in a tetrodotoxin (1 micromol/L)- and guanethidine (10 micromol/L)-sensitive manner. The EFS-evoked overflow of NA in veins exceeded the overflow of NA in arteries at all frequencies of stimulation, whereas the EFS-evoked overflow of ATP, ADP and AMP in veins exceeded the overflow of adenine nucleotides in arteries at 4 and 8 Hz but not at 16 Hz stimulation. The EFS-evoked overflow of adenosine was similar in arteries and veins. 4. Activation of alpha1-adrenoceptors with methoxamine (10 micromol/L) did not produce overflow of ATP. 5. Blockade of alpha1/alpha2-adrenoceptors with phentolamine (1 micromol/L) did not affect EFS-evoked overflow of ATP, ADP, AMP and adenosine. 6. It is concluded that overflow of ATP and NA from sympathetic nerves may constitute an effective mechanism in the complex balance between capacitance and resistance in splanchnic circulation.