Lowering of the extracellular Na⁺ concentration enhances high-K⁺-induces formation of inositol phosphates in the guinea-pig ileum

1. Formation of inositol phosphates (InsPs) was measured in cross-chopped slices or dispersed cells, isolated by collagenase treatment, of guinea-pig ileum longitudinal smooth muscle pre-labelled with [³H]inositol. 2. Elevation of the extracellular K⁺ concentration by equimoler replacement of Na⁺ induced accumulation of InsPs in the dispersed cells and in the tissue slices. These effects were blocked by neither tetrodotoxin (1 μM) nor atropine (10 μM), and were approximately additive with carbachol-induced accumulation. 3. In the tissue slices, the response to K⁺ was partially inhibited by nifedipine (10 μM) and by CdCl₂ (0.3 mM), but the carbachol-induced response was not altered. 4. Accumulation of InsPs induced by KCl-excess solution (high-K⁺ solution without Na⁺ replacement) was suppressed strongly by nifedipine and completely by CdCl₂. The response to KCl excess was approx. 40% of that to high K⁺ with Na⁺ replacement. 5. Low-NaCl solution (replacement of NaCl with equimolar sucrose) also produced InsPs, and this was not blocked by either nifedipine (10 μM) or CdCl₂ (0.3 mM). 6. The formation of InsPs by a maximally effective concentration of carbachol (1 mM) in the presence of KCl excess or low NaCl was greater than the additive effect of the two stimuli on their own. Enhancement of the carbachol-induced response by KCl excess disappeared in the presence of CdCl₂ (0.3 mM). 7. These data suggest that formation of InsPs induced by high-K⁺ solution with equimolar replacement of Na⁺ consist of two components, i.e. high-K⁺-induced inositol-phospholipid hydrolysis by Ca²⁺ entry through voltate-sensitive channels, and low-Na⁺-induced formation of InsPs, insensitive to Ca²⁺ antagonists, but that both of them do not contribute significantly to the activation of phospholipase C by muscarinic stimuli.