ABSTRACT
1. In chicken hepatocytes, alpha 1-adrenoceptor activation increased: (a) phosphatidylinositol labeling; (b) production of inositol trisphosphate; (c) cytosol calcium; and (d) phosphorylase activity. 2. Prazosin (Ki approximately 0.2-0.4 nM) was more potent in inhibiting these actions than 5-methyl-urapidil (Ki approximately 30-60 nM); these actions were sensitive to chlorethylclonidine suggesting the involvement of alpha 1B-adrenoceptors. 3. The stimulation of phosphoinositide turnover was insensitive to pertussis toxin. 4. In chicken liver membranes, [3H]prazosin binding sites (Bmax 872 fmol/mg protein) with high affinity for prazosin (KD 0.3 nM; Ki 0.4 nM) and lower affinity for 5-methyl-urapidil (Ki 46 nM) were detected, consistent with the presence of alpha 1B-adrenoceptors.
Subject(s)
Liver/metabolism , Receptors, Adrenergic, alpha-1/metabolism , Signal Transduction/physiology , Adrenergic alpha-Antagonists/pharmacology , Animals , Calcium/metabolism , Chickens , Epinephrine/pharmacology , In Vitro Techniques , Membranes/drug effects , Membranes/metabolism , Norepinephrine/pharmacology , Pertussis Toxin , Phenylephrine/pharmacology , Phosphatidylinositols/metabolism , Piperazines/pharmacology , Prazosin/metabolism , Receptors, Adrenergic, alpha-1/drug effects , Thermodynamics , Virulence Factors, Bordetella/pharmacologyABSTRACT
The effect of 12-tetradecanoyl phorbol 13-acetate on the GTPase activity of Gi was investigated. Treatment with TPA did not alter basal GTPase activity of membranes or the stimulatory effect of prostaglandin E1 (putatively via Gs). In contrast, the phorbol ester markedly diminished stimulation of GTPase by agents whose receptors are coupled to Gi such as epinephrine (alpha-adrenergic action), platelet activating factor or thrombin. Pertussis toxin catalyzed ADP-ribosylation was also decreased in membranes from TPA-treated platelets as compared to the controls. It is suggested that the alteration in the hormonal activation of the GTPase activity of Gi is secondary to a perturbation in the receptor-Gi interaction.
Subject(s)
Blood Platelets/enzymology , GTP Phosphohydrolases/metabolism , GTP-Binding Proteins/metabolism , Protein Kinase C/metabolism , Adenosine Diphosphate Ribose/metabolism , Alprostadil/pharmacology , Enzyme Activation/drug effects , Epinephrine/pharmacology , Humans , Pertussis Toxin , Platelet Activating Factor/pharmacology , Tetradecanoylphorbol Acetate/pharmacology , Thrombin/pharmacology , Virulence Factors, Bordetella/pharmacologyABSTRACT
The effect of 12-tetradecanoyl phorbol 13-acetate (TPA) on the hormonal modulation of adenylate cyclase was studied. The effect of epinephrine (alpha 2-adrenergic action) was markedly diminished in membranes from TPA-treated platelets as compared to the controls. Interestingly, the inhibitory effect of guanylyl imido diphosphate (Gpp(NH)p) was not altered. Neither the number of alpha 2-adrenoceptors nor their affinity for [3H]yohimbine were affected by the treatment with TPA. In control platelets, 77% of the receptors were in a high-affinity state for epinephrine and 22% in a low-affinity state; Gpp(NH)p shifted the receptor affinity towards the low-affinity conformation. In membranes from TPA-treated platelets, the receptors were in the low-affinity state and no further decrease in affinity was induced by Gpp(NH)p. Our data suggest that activation of protein kinase C in platelets blocks the hormonal inhibition of adenylate cyclase by interfering with the receptor-Gi interaction.
