Biomedical subjects

S B Masters

Publications and source records attributed to S B Masters.

22 records · Page 2Linked to original sources

PubMed1984

Muscarinic regulation of phosphatidylinositol turnover and cyclic nucleotide metabolism in the heart.

Stimulation of cardiac muscarinic receptors leads to increases in the synthesis and hydrolysis of the membrane phospholipid phosphatidylinositol (PI). Carbachol stimulates PI hydrolysis in right and left murine atria as well as in murine ventricule and dissociated embryonic chick heart cells. Muscarinic stimulation of PI hydrolysis is markedly attenuated in calcium-free medium, is not antagonized by isoproterenol, occurs after a latency of several minutes, and is half-maximally activated by approximately 10 microM carbachol. In contrast, muscarinic inhibition of cyclic AMP accumulation in the same preparations is calcium independent, is opposed by the effect of isoproterenol, is maximal in minutes, and is half-maximally activated by 0.1 microM carbachol. These differences demonstrate that the two muscarinic receptor-mediated events are probably unrelated and independent responses. The concentration of carbachol that causes half-maximal activation of PI hydrolysis is almost identical to that causing half muscarinic receptor occupancy as assessed by 3H-labeled (-)-quinuclidinyl benzilate binding. Thus activation of the PI response by carbachol appears to be closely linked to receptor occupancy, whereas cyclase inhibition may occur when only a small percentage of receptors are occupied. The possible role of the PI response in generating intracellular signals such as arachidonic acid release, cyclic GMP synthesis, or C-kinase activation is discussed.

Acetylcholine↗

PubMed1984

Relationships between phosphoinositide and calcium responses to muscarinic agonists in astrocytoma cells.

Activation of muscarinic receptors in human astrocytoma (1321N1) cells stimulates phosphoinositide metabolism and calcium mobilization. The muscarinic effect on phosphoinositide turnover is evidenced by increased formation of [3H]inositol 1-phosphate (Ins1P) and by increased [3H]inositol incorporation into PtdIns. The muscarinic effect on calcium mobilization is seen as a large increase in undirectional 45Ca2+ efflux from cells equilibrated with 45Ca2+ and a small increase in unidirectional 45Ca2+ influx. A series of muscarinic agonists was used to explore the relationship between phosphoinositide metabolism and unidirectional 45Ca2+ efflux. The maximal increases in [3H]Ins1P formation produced by carbachol and acetylcholine are similar and are much larger than those caused by oxotremorine and pilocarpine. The effects of these agonists on 45Ca2+ efflux are similar: carbachol and acetylcholine cause equivalent maximal increases in the rate of 45Ca2+ efflux whereas oxotremorine and pilocarpine cause submaximal 45Ca2+ efflux responses. The Kact values of carbachol and acetylcholine for stimulation of [3H]Ins1P formation are 40 microM and 1.5 microM, respectively. These values are only 2- to 3-fold higher than the respective Kact values for stimulating 45Ca2+ efflux. The finding that each of the muscarinic agonists tested has nearly identical efficacy and similar potency for stimulating [3H]Ins1P formation and 45Ca2+ efflux supports the idea that hormonal stimulation of phosphoinositide hydrolysis leads to calcium mobilization.

Astrocytoma↗

PubMedDate not supplied

Identification of receptor contact site involved in receptor-G protein coupling.

The mammalian G proteins transduce information from extracellular signals, including neurotransmitters, hormones and sensory stimuli, into regulation of effector enzymes or ion channels within cells. Triggered by appropriate extracellular signals, receptor proteins specifically activate members of the G protein family by catalysing replacement of GDP by GTP at the guanine nucleotide binding site. Like the receptor proteins, the heterotrimeric G proteins exhibit impressive structural similarities, suggesting that all receptor-G protein interactions use homologous structural elements and a single molecular mechanism. Topologically equivalent portions of each G protein may therefore interact with the appropriate receptor. We recently predicted the secondary structure of a composite G protein alpha-chain and proposed that a predicted amphipathic alpha-helix at the extreme carboxy-terminus of the polypeptide directly contacts receptors. This proposal has now been confirmed by sequencing complementary DNAs of the gene that encodes the alpha-chain (alpha s) of the stimulatory regulator (Gs) of adenylyl cyclase in wild-type cells and in a mutant mouse S49 lymphoma cell line, unc, in which Gs cannot be activated by hormone receptors. The sequences reveal a point mutation in the unc gene that substitutes a proline residue for an arginine near the carboxy-terminus of the alpha s-polypeptide. Expression of recombinant alpha s-unc in genetically alpha s-deficient S49 cells reproduces the unc phenotype.

Amino Acid Sequence↗

PubMedDate not supplied

Family of G protein alpha chains: amphipathic analysis and predicted structure of functional domains.

The G proteins transduce hormonal and other signals into regulation of enzymes such as adenylyl cyclase and retinal cGMP phosphodiesterase. Each G protein contains an alpha subunit that binds and hydrolyzes guanine nucleotides and interacts with beta gamma subunits and specific receptor and effector proteins. Amphipathic and secondary structure analysis of the primary sequences of five different alpha chains (bovine alpha s, alpha t1 and alpha t2, mouse alpha i, and rat alpha o) predicted the secondary structure of a composite alpha chain (alpha avg). The alpha chains contain four short regions of sequence homologous to regions in the GDP binding domain of bacterial elongation factor Tu (EF-Tu). Similarities between the predicted secondary structures of these regions in alpha avg and the known secondary structure of EF-Tu allowed us to construct a three-dimensional model of the GDP binding domain of alpha avg. Identification of the GDP binding domain of alpha avg defined three additional domains in the composite polypeptide. The first includes the amino terminal 41 residues of alpha avg, with a predicted amphipathic alpha helical structure; this domain may control binding of the alpha chains to the beta gamma complex. The second domain, containing predicted beta strands and alpha helices, several of which are strongly amphipathic, probably contains sequences responsible for interaction of alpha chains with effector enzymes. The predicted structure of the third domain, containing the carboxy terminal 100 amino acids, is predominantly beta sheet with an amphipathic alpha helix at the carboxy terminus. We propose that this domain is responsible for receptor binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗