Activation of α- and β-adrenoceptors by sympathetic nerve stimulation in the large intestine of the rat

1. The effects of sympathetic nerve stimulation on the motility of the circular and longitudinal muscle of the large intestine were investigated in vitro, and the involvement of various adrenoceptor subtypes determined. A comparison between the sympathetic supply arising from the prevertebral and pelvic ganglia was also made. 2. In the longitudinal muscle of the distal colon, sympathetic nerve stimulation caused responses which were contractile (0.1-2 Hz), biphasic (5-10 Hz) or purely inhibitory (20-30 Hz). All contractile responses were removed with phentolamine (3 mu M), whereas the inhibitory responses were significantly diminished by propranolol (0.1 mu M) and completely abolished by alprenolol (3 mu M) or nadolol (300 mu M). 3. In the longitudinal muscle of the proximal colon, the effects of sympathetic nerve stimulation were predominantly inhibitory. Some of this inhibition was removed by propranolol (0.1 mu M), but was largely unaffected by alprenolol(3 mu M). The remainder of the inhibitory response was probably non-noradrenergic as it was not removed by a combination of phentolamine (3 mu M) and alprenolol (3 mu M). 4. In the circular muscle of both the proximal and distal colon, sympathetic stimulation caused a strong contractile response which was completely removed by phentolamine (3 mu M) to reveal an inhibitory response. This inhibitory response was unchanged by propranolol (0.1 mu M) but was removed by alprenolol (3 mu M), following which a weak non-noradrenergic contractile response was unmasked. 5. Stimulation of the hypogastric nerve to activate pelvic sympathetic pathways had no effect on the motility of the longitudinal muscle, but caused a contractile response in the circular muscle which was completely removed by phentolamine (3 mu M). 6. We conclude that sympathetic nerves innervate adrenoceptors of different types in the various muscle layers and regions of the colon. They innervate a mixture of alpha-, and beta(3)-adrenoceptors in the longitudinal muscle of the proximal colon, alpha-, classical beta- and beta(3)-adrenoceptors in the distal colon, and primarily alpha-adrenoceptors with a few beta(3)-adrenoceptors in the circular muscle. In addition, the pelvic sympathetic innervation of the rectum differs from the prevertebral supply by innervating only excitatory alpha-adrenoceptors.