Regional Differences in Calcium Sensitivity in the Guinea-pig Intestine

The effect of the Ca²⁺-channel antagonist nicardipine on the basal tone of six segments (duodenum, jejunum, ileum, proximal colon, distal colon and rectum) of the guinea-pig intestine has been investigated in muscle preparations. Nicardipine reduced the basal tone of the proximal and distal colon but not of the duodenum, jejunum, ileum and rectum. Similarly, when each segment was incubated in Ca²⁺-free medium, the basal tone of the proximal and distal colon, but not that of the other four segments, was reduced. The reduced basal tone recovered after cumulative addition of Ca²⁺ in both colon preparations. The basal tone of the distal colon was partly reduced by tetrodotoxin, atropine and clonidine. Conversely, l -typeCa²⁺-channel antagonists (Cd²⁺, verapamil and nicardipine), but not the T-type Ca²⁺-channel antagonist Ni²⁺, completely reduced the basal tone of the distal colon. These results indicate that in the regulation of basal tone there are additional regional differences in the effect of Ca²⁺ influx into the cells from the extracellular fluid which might involve l -type-like Ca²⁺ channels and might partly be because of neuronal factors.