Tolbutamide stimulates exocytosis of glucagon by inhibition of a mitochondrial-like ATP-sensitive K+ (KATP) conductance in rat pancreatic A-cells

J Physiol. 2000 Aug 15;527 Pt 1(Pt 1):109-20. doi: 10.1111/j.1469-7793.2000.00109.x.


1. Capacitance measurements were used to examine the effects of the sulphonylurea tolbutamide on Ca2+-dependent exocytosis in isolated glucagon-secreting rat pancreatic A-cells. 2. When applied extracellularly, tolbutamide stimulated depolarization-evoked exocytosis 4.2-fold without affecting the whole-cell Ca2+ current. The concentration dependence of the stimulatory action was determined by intracellular application through the recording pipette. Tolbutamide produced a concentration-dependent increase in cell capacitance. Half-maximal stimulation was observed at 33 microM and the maximum stimulation corresponded to a 3.4-fold enhancement of exocytosis. 3. The stimulatory action of tolbutamide was dependent on protein kinase C activity. The action of tolbutamide was mimicked by the general K+ channel blockers TEA (10 mM) and quinine (10 microM). A similar stimulation was elicited by 5-hydroxydecanoate (5-HD; 10 microM), an inhibitor of mitochondrial ATP-sensitive K+ (KATP) channels. 4. Tolbutamide-stimulated, but not TEA-induced, exocytosis was antagonized by the K+ channel openers diazoxide, pinacidil and cromakalim. 5. Dissipating the transgranular K+ gradient with nigericin and valinomycin inhibited tolbutamide- and Ca2+-evoked exocytosis. Furthermore, tolbutamide- and Ca2+-induced exocytosis were abolished by the H+ ionophore FCCP or by arresting the vacuolar (V-type) H+-ATPase with bafilomycin A1 or DCCD. Finally, ammonium chloride stimulated exocytosis to a similar extent to that obtained with tolbutamide. 6. We propose that during granular maturation, a granular V-type H+-ATPase pumps H+ into the secretory granule leading to the generation of a pH gradient across the granular membrane and the development of a positive voltage inside the granules. The pumping of H+ is facilitated by the concomitant exit of K+ through granular K+ channels with pharmacological properties similar to those of mitochondrial KATP channels. Release of granules that have been primed is then facilitated by the addition of K+ channel blockers. The resulting increase in membrane potential promotes exocytosis by unknown mechanisms, possibly involving granular alkalinization.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Calcium / metabolism
  • Cell Culture Techniques
  • Electric Conductivity
  • Exocytosis / drug effects*
  • Glucagon / metabolism*
  • Hydrogen-Ion Concentration / drug effects
  • Ionophores / pharmacology
  • Islets of Langerhans / drug effects
  • Islets of Langerhans / metabolism*
  • Islets of Langerhans / physiology*
  • Male
  • Membrane Potentials
  • Membrane Proteins / antagonists & inhibitors
  • Membrane Proteins / metabolism*
  • Models, Biological
  • Pituitary Gland / cytology
  • Pituitary Gland / drug effects
  • Pituitary Gland / metabolism
  • Potassium / metabolism
  • Potassium Channels
  • Protein Kinase C / antagonists & inhibitors
  • Proton-Translocating ATPases / antagonists & inhibitors
  • Rats
  • Rats, Inbred Lew
  • Rats, Sprague-Dawley
  • Sulfonylurea Compounds / pharmacology
  • Tolbutamide / pharmacology*
  • Vacuolar Proton-Translocating ATPases*


  • Ionophores
  • Membrane Proteins
  • Potassium Channels
  • Sulfonylurea Compounds
  • mitochondrial K(ATP) channel
  • Glucagon
  • Tolbutamide
  • Protein Kinase C
  • Vacuolar Proton-Translocating ATPases
  • Proton-Translocating ATPases
  • Potassium
  • Calcium