Substance P inhibits activation of calcium-dependent potassium conductances in guinea-pig myenteric neurones.
1. Intracellular recordings were made from myenteric AH neurones of the guinea-pig ileum in vitro. Some experiments were done with a single-electrode voltage clamp to measure membrane currents. 2. Substance P (SP) applied by superfusion (10 nM-300 nM), pressure ejection (100 nM-10 microM, 760 mmHg, for 10-20 ms) or ionophoresis (1 mM, 100 nA, for 0.2 s) caused a membrane depolarization and an inward current, associated with a decrease in potassium conductance. 3. The SP-induced depolarization was abolished within 15 min by superfusion with calcium-free/high-magnesium (10 mM) solution or solutions containing cobalt, manganese or nickel at 1-3 mM. The response persisted even after 40-60 min of superfusion with calcium-free/normal-magnesium (1.2 mM) solution. In all these solutions, synaptic potentials were abolished within 5 min. 4. SP inhibited a slowly developing outward current and an outward tail current during and after a long depolarizing command pulse (2-10 s), and an outward after-current following single or multiple brief depolarizing command pulses (10-50 ms). These outward currents were suppressed in calcium-free/high-magnesium solution. 5. SP depressed both a calcium-dependent slow after-hyperpolarization following the action potential and an outward after-current preceded by a brief depolarizing command. Both the SP-induced depolarization and the SP-induced inward current were augmented when the peptide was pressure-ejected during the recovery phase of the slow after-hyperpolarization and during that of the slow outward after-current, but both of them were inhibited or almost abolished when SP was applied immediately after spike initiation or a brief depolarizing command. 6. The SP-induced response was depressed by barium (1-2 mM). The SP response was not inhibited by tetraethylammonium at low concentrations (5-10 mM), but was depressed at high concentration (20 mM). 7. Superfusion (1-10 nM) or pressure application of a calcium ionophore, A23187, inhibited or even reversed the SP depolarization and the SP-induced inward current. 8. These results indicate that SP inhibits activation of a calcium-dependent potassium conductance which contributes to both the slow after-hyperpolarization and the resting membrane potential. SP may affect the process by which calcium activates this potassium conductance.
Morita
K
K
Department of Autonomic Physiology, Tokyo Medical and Dental University, Japan.
Katayama
Y
Y
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000493:pharmacokinetics*; D006168:Guinea Pigs; D007082:Ileum / Q000294:innervation; D008564:Membrane Potentials / Q000187:drug effects; D009197:Myenteric Plexus / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D011188:Potassium / Q000493:pharmacokinetics*; D013373:Substance P / Q000494:pharmacology*
D001464:Barium; D013373:Substance P; D000001:Calcimycin; D011188:Potassium; D002118:Calcium