A comparison between muscarinic receptor occupancy, inositol 1,4,5-trisphosphate accumulation and Ca 2+ mobilization in permeabilized SH-SY5Y neuroblastoma cells

Abstract

Electrically permeabilized SH-SY5Y neuroblastoma cells have been used to examine the relationship between receptor occupation by muscarinic agonists, inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3) accumulation and Ca 2+ mobilization from intracellular stores. The kinetics, concentration-dependence and guanine nucleotide-sensitivity of these responses have been characterized for the agonists, carbachol, arecoline and oxotremorine. Carbachol stimulated Ins(1,4,5)P 3 accumulation and Ca 2+ mobilization with an EC 50 value ∼50 μM, only slightly lower than the apparent affinity of this agonist for the “free” receptor (100 μM). Arecoline and oxotremorine were partial agonists, mobilizing 45 and 21% of the Ca 2+ mobilized by carbachol, and yielded EC 50 values for both Ins(1,4,5)P 3 and Ca 2+ responses, similiar to their binding affinity. Guanosine 5'-O-3 thio-triphosphate (GTPγS) markedly enhanced the responses elicited by all three agonists. Carbachol became significantly more potent for both Ins(1,4,5)P 3 accumulation (EC 50 = 4.1 μM) and Ca 2+ mobilization (EC 50 = 0.25 μM), revealing a separation of the dose-response relationships. GTPγS caused a smaller separation of the responses elicited by arecoline (Ca 2+ mobilization EC 50 = 0.9 μM; Ins(1,4,5)P 3 accumulation EC 50 = 3.6 μM), and only enhanced maximal responses for oxotremorine. These data reveal that the functional coupling of muscarinic receptors to activation of phosphoinositidase C and subsequent Ca 2+ mobilization from intracellular stores is maintained after electrical permeabilization. Furthermore, this model has been used to reveal differences in the relative activities of muscarinic agonists and how they are influenced by a hydrolysis-resistant guanine nucleotide.