Functional overlap of IP₃- and cADP-ribose-sensitive calcium stores in guinea pig myenteric neurons

In myenteric neurons two different receptor subtypes govern the intracellular Ca²⁺ stores: the inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and the ryanodine receptor (RyR). Their degree of functional overlap was determined by examining Ca²⁺ release in these cells through both superfusion techniques and intracellular microinjection. Microinjection of IP₃ (50 μM) and cADP-ribose (cADPr, 50 μM), specific ligands for the IP₃R and RyR, respectively, demonstrated mobilization of intracellular Ca²⁺ stores. Perfusion with cinnarizine (50 μM) or dantrolene (10 μM), antagonists of the IP₃R and RyR, respectively, eliminated the Ca²⁺ response to microinjected IP₃ and cADPr. Superfusion of the neurons with 100 μM ATP, an IP₃-mediated Ca²⁺-mobilizing agonist, caused intracellular Ca²⁺ increments, which were antagonized by cinnarizine, and the RyR antagonists dantrolene, procaine (5 mM), and ryanodine (1 μM). Caffeine (10 mM) was applied repetitively in Ca²⁺-free conditions to deplete RyR-sensitive stores; subsequent perfusion with ATP demonstrated a Ca²⁺ response. Conversely, caffeine caused a Ca²⁺ response after repetitive ATP exposures. The internal Ca²⁺ stores of myenteric neurons are governed by two receptor subtypes, IP₃R and RyR, which share partial functional overlap.