Details

Autor(en) / Beteiligte

• THAYER, STANLEY ARLEN

Titel

A SEARCH FOR AN ENDOGENOUS MODULATOR OF CALCIUM CHANNELS (ENTRY BLOCKERS)

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1985

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• This dissertation describes an investigation into the possible existence of an endogenous modulator of the calcium channel. The rationale for searching for such a factor is based primarily on three observations: the electrophysiological phenomenon of calcium channel rundown, the receptor-like nature of the calcium entry blocking drug binding sites, and the ability to improve the binding of tritiated calcium entry blockers to brain membranes by repeated washing. The hypothesis, that an endogenous modulator of the calcium channel exists, was tested by searching for such a factor. Extraction of brain tissue followed by gel filtration chromatography produced a partially purified extract that may contain the proposed modulator, which was tested in calcium entry blocker radioligand binding assays and in isolated smooth muscle preparations. The brain extract was tested for its ability to fulfill four criteria which define the characteristics of an endogenous modulator of the calcium channel; these criteria and the results of the experiments to test them are as follows. (1) The modulator should be soluble and relatively small since its function requires that it move on and off the receptor it regulates. This criterion was fulfilled as indicated by a molecular weight of less than 13,000 Daltons, as determined by gel filtration chromatography. (2) The substance should produce a physiological response, namely, an effect at the calcium channel. The partially purified fraction from gel filtration chromatography produced a dose dependent contraction of rabbit aorta which was potentiated by subthreshold depolarization and blocked by 0.1 (mu)M nifedipine. These results suggest that the factor exerts its effects by the activation of plasmalemmal calcium channels. (3) While not an absolute requirement, one might expect the modulator to be affected by factors which affect the calcium channel, such as calcium and the calcium entry blockers. The extract inhibited the binding of ('3)H nitrendipine and ('3)H verapamil to skeletal muscle membranes by a mixed competitive-noncompetitive and a competitive mechanism respectively. The inhibition required calcium. (4) The effects produced by the modulator should be reversible. It is not clear whether this criterion was satisfied since the factor produced a contraction in the aorta that was readily reversible, while the inhibition of the binding assays was not. This dissertation describes the discovery of a substance in tissue extracts which may act as a modulator of calcium channels. Definitive proof that such a substance exists will require its purification to homogeneity and structural determination.