Substance P induces inward current and regulates pacemaker currents through tachykinin NK₁ receptor in cultured interstitial cells of Cajal of murine small intestine

We investigated whether substance P modulates pacemaker currents generated in cultured interstitial cells of Cajal of murine small intestine using whole cell patch-clamp techniques at 30°C. Interstitial cells of Cajal generated spontaneous inward currents (pacemaker currents) at a holding potential of -70 mV. Tetrodotoxin, nifedipine, tetraethylammonium, 4-aminopyridine, or glibenclamide did not change the frequency and amplitude of pacemaker currents. However, divalent cations (Ni²⁺, Mn²⁺, Cd²⁺, and Co²⁺), nonselective cationic channel blockers (gadolinium and flufenamic acid), and a reduction of external Na⁺ from normal to 1 mM inhibited pacemaker currents indicating that nonselective cation channels are involved in their generation. Substance P depolarized the membrane potential in current clamp mode and produced tonic inward pacemaker currents with reduced frequency and amplitude in voltage clamp mode. [D-Arg¹, D-Trp ^7,9, Leu¹¹] substance P, a tachykinin NK₁ receptor antagonist, blocked these substance P-induced responses. Furthermore, [Sar⁹, Met(O₂)¹¹] substance P, a specific tachykinin NK₁ receptor agonist, depolarized the membrane and tonic inward currents mimicked those of substance P. Substance P continued to produce tonic inward currents in external Ca²⁺-free solution or in the presence of chelerythrine, a protein kinase C inhibitor. However, substance P-induced tonic inward currents were blocked by thapsigargin, a Ca ²⁺-ATPase inhibitor in the endoplasmic reticulum or by an external 1 mM Na⁺ solution. Our results demonstrate that substance P may modulate intestinal motility by acting on the interstitial cells of Cajal by activating nonselective cation channels via the release of intracellular Ca ²⁺ induced by tachykinin NK₁ receptor stimulation.