Regulation of calcium-activated nonselective cation channel activity by cyclic nucleotides in the rat insulinoma cell line,CRI-G1

The regulation of a calcium-activated nonselective cation (Ca-NS⁺) channel by analogues of cyclic AMP has been investigated in the rat insulinoma cell line, CRI-G1. The activity of the channel is modulated by cyclic AMP in a complex way. In the majority of patches (83%) tested concentrations of cyclic AMP of 10 μm and above cause an inhibition of channel activity which is immediately reversible on washing. In contrast, lower concentrations of cyclic AMP, between 0.1 and 1.0 μm, produce a transient activation of channel activity in most patches (63%) tested. One group of analogues, including N⁶-monobutyryl cyclic AMP and N⁶, 2′-O-dibutyryl cyclic AMP reduced the activity of the Ca-NS⁺ channel at all concentrations tested and 2′-O-Monobutyryl cyclic AMP produced inhibition in all patches tested except one, at all concentrations. A second group produced dual concentration-dependent effects on Ca-NS⁺, low concentrations stimulating and high concentrations inhibiting channel activity. 6-Chloropurine cyclic AMP and 8-bromo cyclic AMP produced effects similar to those of cyclic AMP itself. In contrast, 8-[4-chlorophenylthio] cyclic AMP also showed a dual action, but with a high level of activation at all concentrations tested up to 1mm. Ca-NS⁺ channel activity was also predominantly activated by low concentrations of S_p-cAMPS. The activating effects of both S_p-cAMPS and cyclic AMP are antagonized by R_p-cAMPS, which by itself only produced a weak inhibition of Ca-NS⁺ channel activity even at concentrations of 10 μm and above. The results are discussed in terms of a model in which cyclic AMP, and other cyclic nucleotides, modulate the activity of the Ca-NS⁺ channel by binding to two separate sites.