T-type Ca2+ channels contribute to IBMX/forskolin- and K+-induced Ca2+ transients in porcine olfactory receptor neurons

T-type Ca2+ channels are low-voltage-activated Ca2+ channels that control Ca2+ entry in excitable cells during small depolarization above resting potentials. Using Ca2+ imaging with a laser scanning confocal microscope we investigated the involvement of T-type Ca2+ channels in IBMX/forskolin- and sparingly elevated extracellular K+-induced Ca2+ transients in freshly isolated porcine olfactory receptor neurons (ORNs). In the presence of mibefradil (10 μM) or Ni2+ (100 μM), the selective T-type Ca2+ channel inhibitors, IBMX/forskolin-induced Ca2+ transients in the soma were either strongly (>60%) inhibited or abolished completely. However, the Ca2+ transients in the knob were only partially (<60%) inhibited. Ca2+ transients induced by 30 mM K+ were also partially (∼60%) inhibited at both the knob and soma. Furthermore, ORNs responded to as little as a 2.5 mM increase in the extracellular K+ concentration (7.5 mM K+), and such responses were completely inhibited by mibefradil or Ni2+. These results reveal functional expression of T-type Ca2+ channels in porcine ORNs, and suggest a role for these channels in the spread Ca2+ transients from the knob to the soma during activation of the cAMP cascade following odorant binding to G-protein-coupled receptors on the cilia/knob of ORNs.