Regional differences in calcium sensitivity in the guinea-pig intestine

The effect of the Ca(2+)-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 Ca(2+)-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-type Ca(2+)-channel antagonists (Cd2+, verapamil and nicardipine), but not the T-type Ca(2+)-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.