Somatostatin is a powerful inhibitor of intestinal Cl secretion. We used patch-clamp recording techniques to investigate the effects of somatostatin on low-conductance (23-pS) K⁺ channels in the basolateral membrane of human colonic crypts, which are an important component of the Cl secretory process. Somatostatin (2 µM) elicited a >80% decrease in "spontaneous" K⁺ channel activity in cell-attached patches in nonstimulated crypts (50% inhibition =~8 min), which was voltage-independent and was prevented by pretreating crypts for 18 h with pertussis toxin (200 ng/ml), implicating a G protein-dependent mechanism. In crypts stimulated with 100-200 µM dibutyryl cAMP, 2 µM somatostatin and its synthetic analog octreotide (2 µM) both produced similar degrees of K⁺ channel inhibition to that seen in nonstimulated crypts, which was also present under low-Cl (5 mM) conditions. In addition, 2 µM somatostatin abolished the increase in K⁺ channel activity stimulated by 2 µM thapsigargin but had no effect on the thapsigargin-stimulated rise in intracellular Ca²⁺. These results indicate that somatostatin peptides inhibit 23-pS basolateral K⁺ channels in human colonic crypt cells via a G protein-dependent mechanism, which may result in loss of the channel's inherent Ca²⁺ sensitivity.