ALPHA(2)-ADRENERGIC MODULATION OF COLONIC TONE DURING HYPERVENTILATION

Our aims were to assess the role of adrenergic modulation in the hyper ventilation-induced increase in colonic tone. Of 40 healthy volunteers , 12 received placebo (saline) and the remaining 28 received either cl onidine, yohimbine, phenylephrine, or ritodrine. Time-frequency mappin g of heart rate based on Wigner distribution assessed variations in pa rasympathetic and sympathetic activity during hyperventilation. Tone i n the descending colon was recorded by a barostat balloon before, duri ng, and after 5 min of hyperventilation. Heart rate spectral analysis suggested diminished sympathetic and vagal activity during hyperventil ation and increased sympathetic and vagal activity after hyperventilat ion. Adrenergic agents influenced (P = 0.01) the tonic response after, but not during, hyperventilation. Yohimbine reduced the increment in colonic tone after hyperventilation compared with saline (P < 0.05) an d clonidine (P = 0.002); phenylephrine and ritodrine had no effects. D ifferent mechanisms modulate the increase in colonic tone during and a fter hyperventilation. Yohimbine attenuates the increase in colonic to ne after hyperventilation probably by enhancing inhibitory sympathetic input to the colon.