Subclasses of angiotensin II binding sites and their functional significance.

Specific binding sites for angiotensin II were identified in the rabbit adrenal gland and uterus and in the rat liver and were divided into two subclasses based on inhibition by nonpeptide ligands. Peptide ligands affected binding equally in all three tissues. However, the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II binding to adrenal and liver homogenates at nanomolar concentrations but exerted only a minimal effect on binding to uterine homogenates. The nonpeptide PD 123319 potently blocked angiotensin II binding to uterine homogenates but had no effect on adrenal or liver homogenates at concentrations up to 10 microM. Further analysis of angiotensin II binding in uterus showed that both sites are present, with the PD 123319-sensitive site predominating. Additionally, the nonhydrolyzable GTP analogue 5'-guanylyl-imidodiphosphate was able to modulate binding to liver and to the Dup 753-sensitive site in uterus but not that to the PD 123319-sensitive site. Saralasin and the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II-stimulated accumulation of inositol phosphates in cultured Clone 9 cells and also relaxed aortic rings previously contracted with angiotensin II. In contrast, PD 123319 had no effect on either angiotensin II-stimulated inositol phosphate accumulation or vasoconstriction. Saralasin and Exp 6803, but not PD 123319, lowered blood pressure in renal hypertensive rats following intravenous administration. These results suggest the existence of two subclasses of angiotensin II binding sites, which differ in their tissue distribution and affinity for the nonpeptide ligands Dup 763, Exp 6803, and PD 123319. Although no functional role for the PD 123319-sensitive subclass has yet been identified, the Dup 753/Exp 6803-sensitive subclass plays an important role in mediating inositol phosphate metabolism, vascular contractile activity, and blood pressure regulation.

Differential inhibition of human neutrophil functions. Role of cyclic AMP-specific, cyclic GMP-insensitive phosphodiesterase.

Multiple molecular forms of cyclic nucleotide phosphodiesterase have been characterized in various tissues and cells according to their substrate specificity, intracellular location, and calmodulin dependence. The purpose of this study was to evaluate the possible involvement of different molecular forms of phosphodiesterase in regulating the respiratory burst and lysosomal enzyme release responses of human neutrophils. Treatment with the selective cyclic AMP-specific, cyclic GMP-insensitive phosphodiesterase inhibitors Ro 20-1724 or rolipram, or the nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), resulted in inhibition of respiratory burst stimulated by the chemoattractants formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) (IC50 values: 0.71-17 microM) and complement fragment C5a (IC50 values: 61-93 microM), but did not inhibit phagocytosis-stimulated respiratory burst (less than 10% inhibition at 100 microM). Selective inhibitors of calmodulin-dependent phosphodiesterase (ICI 74,917), calmodulin-insensitive, cyclic GMP-specific phosphodiesterase (M & B 22,948), cyclic GMP-stimulated phosphodiesterase (AR-L 57), or cyclic AMP-specific, cyclic GMP-inhibited phosphodiesterase (amrinone and cilostamide) exhibited little or no inhibitory effect on FMLP- or phagocytosis-stimulated respiratory burst (0-42% inhibition at 100 microM). Regulation of neutrophil activation by phosphodiesterase was also response specific, as Ro 20-1724, rolipram and IBMX were less potent inhibitors of FMLP-induced lysosomal enzyme release (0-14% inhibition at 100 microM). Analysis of human neutrophil preparations confirmed the existence of a cyclic AMP-specific, cyclic GMP-insensitive phosphodiesterase, which was associated with the particulate fraction of the cell. These results demonstrate a role for the cyclic AMP-specific, cyclic GMP-insensitive phosphodiesterase in the regulation of human neutrophil functions, which appears to be both stimulus specific and response specific.

New mechanisms for positive inotropic agents: focus on the discovery and development of imazodan.

Intense efforts during the last decade to identify a useful positive inotropic agent to replace digitalis for the treatment of congestive heart failure have led to the discovery of several dozen potential substitutes, of which a number are currently undergoing clinical trials. In addition to producing a variety of new therapeutic entities, research in this area has also yielded valuable new information regarding the fundamental events that regulate calcium homeostasis and contractile function in the cardiac cell. For example, several of these new inotropic agents, including the calcium-channel stimulator BAY-k 8644, the sodium-channel stimulator DPI-201-186, and the sodium-calcium exchange inhibitor dichlorobenzamil, have provided considerable insight into the role of sodium and calcium in regulating contractility and the molecular events that mediate potential-dependent ion channels. Likewise, the discovery and development of agents like imazodan, amrinone, enoximone, and other selective type III phosphodiesterase inhibitors have provided new information regarding multiple molecular forms of cyclic nucleotide phosphodiesterase, compartmentation of cyclic AMP, and the importance of soluble vs. membrane-bound phosphodiesterases.