PubMed Health⌕ Search

Biomedical subjects

R J Lefkowitz

Publications and source records attributed to R J Lefkowitz.

At least 469 records · Page 26Linked to original sources
PubMed1980

Differential regulation of the alpha 2-adrenergic receptor by Na+ and guanine nucleotides.

Many hormones interact with receptors which stimulate the enzyme adenylate cyclase. Less well characterized ar those receptors which mediate an inhibition of adenylate cyclase activity. However, guanine nucleotides are clearly important in the regulation of both stimulatory and inhibitory receptors. Monovalent cations, notably Na+, regulate many inhibitory receptor systems but apparently not stimulatory receptors. We investigate here the effects of Na+ and guanine nucleotides on the adenylate cyclase-coupled inhibitory alpha 2-adrenergic receptor of the rabbit platelet. Computer modelling of adrenaline competition curves with 3H-dihydroergocryptine (3H-DHE) indicates that adrenaline induces two distinct affinity states of the alpha 2 receptor--one of higher (alpha 2H) and the other of lower (alpha 2L) affinity. Guanyl-5'-yl-imidodiphosphate (Gpp(NH)p) seems to reduce adrenaline affinity to converting the high-affinity state into the low-affinity form of the receptor. In contrast, Na+ reduces adrenaline affinity at both the high- and low-affinity states of the alpha 2 receptor while preserving receptor heterogeneity. Thus, guanine nucleotides and Na+ differ in the manner by which each reduces agonist affinity for the alpha 2-adrenergic receptor.

Animals↗
PubMed1980

Use of cell fusion techniques to probe the mechanism of catecholamine-induced desensitization of adenylate cyclase in frog erythrocytes.

The catecholamine-sensitive adenylate cyclase system appears to be comprised of at least three components; the beta-adrenergic receptor (R component), the catalytic unit of adenylate cyclase (C component) and a nucleotide regulatory protein (N component), responsible for mediating the effects of guanine nucleotides on the system. Cell fusion techniques were used to investigate the role of these three components in the process of homologous desensitization in the frog erythrocyte. Dicyclohexylcarbodiimide (DCCD) was used to inhibit beta-receptor function in one population of frog erythrocytes, whilst phenyl glyoxal was employed to inactivate the N and C components in a second population of frog erythrocytes. Using Sendai virus to fuse the two types of modified cell, heterologous beta-adrenergic receptor-adenylate cyclase systems were constructed which contained components from each cell type. When beta receptors from cells previously desensitized to catecholamines were coupled to N-C components derived from fresh erythrocytes, the resulting hybrid exhibited a densitized response to isoproterenol. By contrast, when beta-adrenergic receptors from fresh cells were coupled to N-C components derived from desensitized erythrocytes, no decreased responsiveness to isoproterenol was apparent in the hybrid. That this resensitization was the result of the addition of fresh beta-adrenergic receptors was demonstrated in a control experiment. Frog erythrocytes were desensitized simultaneously to catecholamines and prostaglandin E1 and modified with DCCD which inactivates the beta-adrenergic receptor but not the prostaglandin receptor. When fresh beta-adrenergic receptors were supplied by cell fusion to these doubly desensitized erythrocytes, only the beta-adrenergic response was restored to control levels. The response to prostaglandin remained desensitized in the hybrids, indicating that the observed resensitization of catecholamine-stimulated adenylate cyclase activity was specific and was due to the addition of fresh beta-adrenergic receptors. These data suggest that in the frog erythrocyte, homologous desensitization is primarily the result of receptor-related alterations.

Adenylyl Cyclases↗
PubMed1980

A ternary complex model explains the agonist-specific binding properties of the adenylate cyclase-coupled beta-adrenergic receptor.

The unique properties of agonist binding to the frog erythrocyte beta-adrenergic receptor include the existence of two affinity forms of the receptor. The proportion and relative affinity of these two states of the receptor for ligands varies with the intrinsic activity of the agonist and the presence of guanine nucleotides. The simplest model for hormone-receptor interactions which can explain and reproduce the experimental data involves the interaction of the receptor R with an additional membrane component X, leading to the agonist-promoted formation of a high affinity ternary complex HRX. Computer modeling of agonist binding data with a ternary complex model indicates that the model can fit the data with high accuracy under conditions where the ligand used is either a full or a partial agonist and where the system is altered by the addition of guanine nucleotide or after treatment with group-specific reagents, e.g. p-hydroxymercuribenzoate. The parameter estimates obtained indicate that the intrinsic activity of the agonist is correlated significantly with the affinity constant L of the component X for the binary complex HR. The major effect of adding guanine nucleotides is to destabilize the ternary complex HRX from which both the hormone H and the component X can dissociate. The modulatory role of nucleotides on the affinity of agonists for the receptor is consistent with the assumption that the component X is the guanine nucleotide binding site. The ternary complex model was also applied successfully to the turkey erythrocyte receptor system. The model provides a general scheme for the activation by agonists of adenylate cyclase-coupled receptor systems and also of other systems where the effector might be different.

Adenylyl Cyclases↗
PubMed1980

Effects of local anesthetics on guanyl nucleotide modulation of the catecholamine-sensitive adenylate cyclase system and on beta-adrenergic receptors.

Tetracaine and other local anesthetics exert multiple actions on the catecholamine-sensitive adenylate cyclase system of frog erythrocyte membranes. Tetracaine (0.2--20 mM) reduces the responsiveness of adenylate cyclase to (a) guanyl-5'-yl-imidodiphosphate and (b) isoproterenol in the presence of GTP or guanyl-5'-yl-imidodiphosphate. Local anesthetics did not affect (a) basal enzyme activity, and (b) enzyme responsiveness to NaF. Tetracaine inhibited stimulation of adenylate cyclase by guanyl-5'-yl-imidodiphosphate over the whole range of nucleotide concentrations. By contrast, inhibition by tetracaine of isoproterenol activity in the presence of GTP was significant only if GTP concentrations exceeded 10(-7) M. Tetracaine also competitively inhibited binding of both the antagonist [3H]dihydroalprenolol and the agonist [3H]hydroxybenzylisoproterenol to beta-adrenergic receptors. However, it was twice as potent in inhibiting [3H]hydroxybenzylisoproterenol as [3H]dihydroalprenolol binding. The greater potency for inhibition of agonist binding was due to the ability of the anesthetics to promote dissociation of the high-affinity nucleotide sensitive state of the beta-adrenergic receptor induced by agonists. Other local anesthetics mimicked the effects of tetracaine on adenylatecyclase and in dissociating high-affinity agonist-receptor complexes. The other of potency for both processes was dibucaine greater than tetracaine greater than bupivacaine greater than lidocaine which agrees with their relative potencies as local anesthetics. By contrast, a different order of potency was observed for competitive inhibition of [3H]dihydroalprenolol binding: dibucaine greater than tetracaine greater than greater than lidocaine greater than bupivacaine.

Adenylyl Cyclases↗
PubMed1980

Beta adrenergic receptors in lymphocyte subpopulations.

To further evaluate the potential utility of lymphocyte beta adrenergic receptor assays in the study of receptor alterations in human disease, we studied highly purified populations of B and T lymphocytes in peripheral blood to see if differences existed in the concentration or affinity of beta adrenergic receptors and catecholamine-responsive cAMP levels. The mean number of receptors present in particulate fractions of B cells did not differ significantly from the number found in T cells. Similarly, no significant difference in the dissociation constant for (-)[3H]dihydroalprenolol was found. Cyclic adenosine monophosphate (cAMP) accumulation in whole lymphocytes as measured by radioimmunoassay was comparable, although a tendency toward lower basal and stimulated levels in the T cells was evident. The data suggest that differences observed in concentrations of beta adrenergic receptors or catecholamine-responsive cAMP accumulation in lymphocytes from patients with varying illnesses are not likely to be due to differences in the proportions of circulating B and T lymphocytes.

B-Lymphocytes↗