Blocking action of berberine on various receptors in rat anococcygeus muscle

The contractile responses of rat anococcygeus muscle to phenylephrine, acetylcholine and 5-hydroxytryptamine in relation to berberine and to their specific antagonists were studied separately. The effects of phenylephrine and acetylcholine were blocked by berberine as well as by atropine. But the alpha-blocking action of atropine was exhibited only at high concentration. The anococcygeus muscle response to phenylephrine was competitively inhibited by berberine with a PA2 value of 6.4. The contraction induced by acetylcholine was competitively inhibited by atropine (PA2 = 8.7), whereas berberine, which differs basically from atropine, shifted the dose-response curve for acetylcholine non-parallelly to the right with its maximal response reduced concomitantly. Its pD2(1) value was 4.6. This suggests that the antagonistic action of berberine may be exerted non-specifically through a site beyond the M-cholinergic receptor. Berberine also relaxed the contraction of anococcygeus muscle induced by 5-hydroxytryptamine. The contractile responses to histamine and isoprenaline were seen only at high concentration. In addition, the response to isoprenaline was little affected by propranolol in our experiment.