Endogenous modulators of binding of [3H]nitrendipine in extracts of rat brain

Gel-filtration chromatography of neutral extract of rat brain revealed the presence of both large (LF; MW greater than 66,000) and small (SF; MW congruent to 1300) MW factors that inhibit the specific binding of [3H]nitrendipine ([3H]NT) to membranes prepared from rat brain. Neither factor inhibited the specific binding of [3H]NT to membranes prepared from rat heart. LF was heat-sensitive, was destroyed by treatment with trypsin and was not converted to SF by boiling. SF was heat-stable but was destroyed on incubation with Pronase. SF was partially purified by boiling, acid treatment and ion-exchange chromatography on Q-Sepharose. Incubation of membranes with either factor resulted in a decrease in the density of binding sites for [3H]NT with no change in the affinity of the binding sites for [3H]NT. The inhibitory effect of LF was fully reversible and was not affected by increasing the concentration of Ca++ in the binding assay. In contrast, inhibition by SF was not reversible but could be prevented by increasing the concentration of Ca++ or other divalent cations in the assay. The presence of LF during preincubation of membranes with SF attenuated the irreversible inhibition of binding of [3H]NT caused by SF. These results suggest that there are at least two distinct factors in rat brain that are capable of modulating the interaction of [3H]NT with binding sites on voltage-dependent calcium channels in rat brain.