Kenneth M.M. Murphy

Autoradiographic localization of calcium channel antagonist receptors in rat brain with [3H]nitrendipine

In vitro autoradiographic techniques have been used to localize [3H]nitrendipine binding sites in the rat brain. The superficial cerebral cortex, the ventral, lateral and posterior nuclei of the thalamus, the molecular layer of the dentate gyrus, the substantia nigra and the external plexiform layer of the olfactory bulb, all contain high densities of silver grains. The level of binding sites are greatly reduced in areas low in synaptic connections. The corpus callosum, the fimbria, the alveus hippocampi and the dorsal commissure of the fornix all lack specific silver grains, as does the lateral olfactory tract of the olfactory bulb. Specific silver grains are not found in the habenula or the hypothalamus. Grains are not associated with blood vessel profiles. The discrete localizations of [3H]nitrendipine binding sites suggest a specific synaptic role.

Adenosine receptors in rat testes: Labeling with 3Hcyclohexyladenosine

Abstract We have labeled adenosine receptors in rat tissues with 3 HN 6 cyclohexyladenosine ( 3 HCHA) and evaluated the organ distribution and drug specificity of these binding sites throughout the body. Detectable receptor concentrations are restricted to the brain and testes. Brain displays receptor levels of 9 pmoles/g wet weight of tissue while the testes display 2 pmoles/g, with all other tissues examined showing less than detectable specific binding of 3 HCHA. The receptors in these two tissues appear to possess identical pharmacologic profiles with an A 1 specificity for both. No change in the total number of 3 HCHA binding sites is seen in testes of rats which had been hypophysectomized 14 days prior to sacrifice. This may exclude the Leydig cells as a source of adenosine receptors in the rat testes.

Myocardial muscarinic acetylcholine receptor: Choline and tris unmask heterogeneity of antagonist binding sites

Abstract The binding properties of myocardial muscarinic acetylcholine receptors are altered in the presence of choline or Tris. The binding of the antagonist [ 3 H]quinuclidinyl benzilate is reduced in the presence of choline or Tris buffer, when compared to parallel determinations in a physiologic salt solution or phosphate buffer. Scatchard analysis indicates the reduced binding is due to a decrease in the apparent number of receptor sites. Experiments with other organic buffers exclude the possibility that the reduced binding in Tris is due to the absence of sodium ions. In the presence of choline or Tris up to 45% of the receptors are not accessible to [ 3 H]quinuclidinyl benzilate. The remaining sites maintain their high affinity for the antagonist. A heterogeneity of antagonist sites is evident.

Obligatory role of a Tris/choline allosteric site in guanine nucleotide regulation of [3H]-L-QNB binding to muscarinic acetylcholine receptors

Tris and choline reduce the maximal binding capacity (RT) of the muscarinic cholinergic antagonist [3H]-L-quinuclidinyl benzilate ([3H]-L-QNB) to atrial membranes, when compared to control values in physiological salt solution (PBS) or NaPi buffer. Addition of guanine nucleotides (GN) to incubations containing choline or Tris reverses the effect of choline and Tris on RT and restores it to levels determined in NaPi or PBS alone. GN addition fails to alter RT or KD values determined in NaPi or PBS in the absence of choline and Tris. This GN effect follows a nucleotide specificity similar to that of the GN regulatory proteins coupled to adenylate cyclase. Tris or choline are required for the expression of GN regulation of [3H]-L-QNB binding to muscarinic acetylcholine receptors (mAChR). An allosteric site recognizing choline and Tris appears involved in the interaction between the guanine nucleotide regulatory protein and antagonist binding to mAChR.

Calcium channel blockade: possible explanation for thioridazine??s peripheral side effect

The authors show that thioridazine possesses calcium antagonist activity which may relate to its cardiac and sexual side effects. Binding sites associated with voltage-operated calcium channels were labeled by 3H-nitrendipine. Thioridazine influenced this binding of 3H-nitrendipine in a fashion similar to known calcium antagonists such as verapamil. Thioridazine also antagonized potassium-induced, calcium-dependent contractions of rat vas deferens, with similar potency. Thioridazine concentrations that exert calcium channel antagonist effects correspond to blood levels at therapeutic doses.