Activation of alpha- and beta-adrenoceptors by sympathetic nerve stimulation in the large intestine of the rat

Abstract

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 microM), whereas the inhibitory responses were significantly diminished by propranolol (0.1 microM) and completely abolished by alprenolol (3 microM) or nadolol (300 microM). 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 microM), but was largely unaffected by alprenolol (3 microM). The remainder of the inhibitory response was probably non-noradrenergic as it was not removed by a combination of phentolamine (3 microM) and alprenolol (3 microM). 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 microM) to reveal an inhibitory response. This inhibitory response was unchanged by propranolol (0.1 microM) but was removed by alprenolol (3 microM), 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 microM). 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.

Figure 1. Sites of electrical stimulation of sympathetic nerves and their connections with the large intestine

Electrical stimuli were applied to the nerves connecting the prevertebral and pelvic ganglia with the gut at the sites indicated by the jagged arrows. The proximal colon was removed with the colic nerves intact, but not including the coeliaco-mesenteric complex. The distal colon was removed with the inferior mesenteric ganglion attached via the lumbar colonic nerves. For stimulation of sympathetic pathways to the muscle of the rectum, one major pelvic ganglion was left attached via the rectal nerves and the hypogastric nerve was stimulated. Stimulation of the hypogastric nerve activates the sympathetic preganglionic axons, but not the parasympathetic preganglionic axons which enter the major pelvic ganglia via the pelvic nerves (not shown). The major pelvic ganglia also supply the caudal rectum via the cavernous nerves (not shown), but these pathways were not investigated. Effects of stimulating the paravascular nerves on the ileum were also investigated although not shown in this figure.

Figure 2. Examples of the effects of stimulating sympathetic nerves on large intestine motility

In each trace the region and layer of intestine is indicated, along with the period of sympathetic nerve stimulation (SNS). A, proximal colon (PC) longitudinal muscle (LM) stimulated at 20 Hz; B, distal colon (DC) longitudinal muscle stimulated at 2 Hz; C, rectum longitudinal muscle stimulated at 20 Hz; D, proximal colon circular muscle (CM) stimulated at 1 Hz; E, distal colon circular muscle stimulated at 10 Hz; F, distal colon circular muscle stimulated at 10 Hz. The activity was integrated for 10–15 s in the first and second halves of the stimulus period (i.e. either side of the dotted line, referred to as the first and second reading in subsequent figures). The time scale is the same for all traces and is indicated in the first trace. All traces are from tissues precontracted with carbachol (0.5–1.5 μM) except F. In most preparations, high frequency SNS resulted in inhibition of activity (A), a mid-range SNS caused a biphasic response (B) and low frequency SNS resulted in excitation of activity (D). Stimulation of the hypogastric nerve had no effect on the longitudinal muscle of the rectum (C). In the circular muscle carbachol attenuated the inhibition observed in the second half of the stimulus period (E and F from the same animal). Note the carbachol induced activity before the stimulus period in A-E. A lack of spontaneous activity in the circular muscle is observed in F.

Figure 3. Control frequency response curves for nerve stimulation of each gut region

In the longitudinal muscle (LM) the effects of sympathetic nerve stimulation (SNS) are shown as a percentage of the carbachol precontraction, with a positive value representing a contraction in addition to the carbachol response and a negative value representing an inhibition. The first reading (▵) and second reading (^) are the SNS-evoked changes in activity recorded in the first and second halves of the stimulus period, respectively (see individual traces in Fig. 2). In the circular muscle (CM), inhibitory responses are also expressed as a percentage of the carbachol response. Contractile responses in the CM were very variable in size between animals and so were normalized such that the largest SNS-evoked contraction in each animal represented 100%. All points show the mean ±s.e.m. from 4–9 animals. PC, proximal colon; DC, distal colon.

Figure 4. Frequency response curves of different gut regions treated with phentolamine

Nerve-evoked responses are expressed as a percentage of the carbachol response or, in the circular muscle, as a percentage of the maximum response. Continuous lines and filled symbols represent responses without adrenoceptor antagonists and the dotted lines with open symbols represent responses in the presence of phentolamine (3 μM). A-D and G were obtained following precontraction with carbachol (0.5–1.5 μM), while E, F and H were obtained without carbachol. The first and second readings are represented by ▴ and •, respectively. Note that phentolamine removed all contractile effects of sympathetic nerve stimulation and in the colon, but not in the rectum, revealed a sympathetic inhibition. Phentolamine does not change the response to stimulation of the hypogastric nerve in the longitudinal muscle of the rectum. Carbachol delays the onset of SNS-evoked inhibition (e.g. the response to 10 Hz stimulation in C and D is purely contractile, but is biphasic in E and F). Data are means ±s.e.m. from 4–9 animals. * Responses in the presence of phentolamine differ from control (P < 0.05). PC, proximal colon; DC, distal colon, CM, circular muscle; LM, longitudinal muscle.

Figure 5. Effects of β-adrenoceptor antagonists on the sympathetic nerve-evoked responses in the distal colon longitudinal muscle

Nerve-evoked responses are expressed as a percentage of the carbachol induced activity and phentolamine (3 μM) was present in all experiments. A, the effects of different concentrations of propranolol on the second reading are shown: 0, ▪; 0.01 μM, ▿; 0.03 μM, ♦; 1 μM, •. For all stimuli of 2–20 Hz, each concentration of propranolol significantly decreased the effects of nerve stimulation when compared with those responses obtained with phentolamine alone (P < 0.05; except for 0.01 μM propranolol, 10 Hz stimuli). B, the effects of nadolol (300 μM, •) and alprenolol (3 μM, □), which block β₃-adrenoceptors in addition to classical β-adrenoceptors, are compared with the effects of 0.1 μM propranolol (▿), which blocks only classical β-adrenoceptors. At 5, 10 and 20 Hz both nadolol and alprenolol removed significantly more of the sympathetic inhibition than propranolol (P < 0.05). All points represent means ±s.e.m. of 4 animals.

Figure 6. Effects of β-adrenoceptor antagonists on nerve-evoked activity in all colonic preparations

Nerve-evoked effects are expressed as a percentage of the carbachol response in the longitudinal muscle (LM) and as a percentage of the maximum contractile or inhibitory response in the circular muscle (CM). The frequency-response curves were obtained in the presence of phentolamine alone (3 μM, ▪) or in combination with propranolol (0.1 μM, ▵) or alprenolol (3 μM, •). Data are means ±s.e.m. from 4 animals. * Effects with propranolol or alprenolol differed from those with phentolamine alone (P < 0.05). † Points at which alprenolol removed more of the sympathetic inhibition than propranolol (P < 0.05). PC, proximal colon, DC, distal colon.