Structural Requirements and Mechanism of the Pressor Activity of Leu-Val-Val-hemorphin-7, a Fragment of Hemoglobin β-chain in Rats

A rat blood pressure assay was used to perform a structure-activity relationship study (SAR) of Leu-Val-Val-hemorphin-7 (LVV-H7), a fragment of hemoglobin (Hb) P-chain, elucidate the mechanisms of its cardiovascular effects, and test its potential involvement in the presser activity of diaspirin crosslinked Hb (DCLHb), a recently developed Hb-based oxygen carrier. The SAR study revealed that the C-terminal-Arg-Phe-amino acid sequence of LVV-H7 contained the main determinants of the presser activity of this peptide. Drug interaction studies using various inhibitory drugs (e.g., phentolamine, clonidine, etc.) and LVV-H7 showed that the presser effect and tachycardia elicited by LVV-H7 involved the activation of the sympathetic nervous system (SNS). Additional studies using phenytoin (sodium channel blocker), [Tic(7)]H7(5-7)-NH2 (putative antagonist of receptors for LVV-H7) and H7(5-7)-NH2, an amidated C-terminal fragment of LVV-H7, suggested that LVV-H7 activated the SNS by interacting with specific receptors functionally coupled with phenytoin-sensitive sodium channels. The presser effect and tachycardia caused by LVV-H7 were potentiated by captopril, suggesting that the angiotensin converting enzyme may contribute to the inactivation of LVV-H7 in rats. The presser activity of DCLHb, in contrast to that elicited by LVV-H7, was not affected by animal pretreatment with LVV-H7 fragments shown to inhibit the presser effect of LVV-H7. We conclude that: 1) LVV-H7 is unlikely to mediate the presser activity of DCLHb in rats; 2) the presser and tachycardic activities of LVV-H7 are mediated by the SNS; 3) the C-terminal-Arg-Phe-amino acid sequence of LVV-H7 contains the chemical groups responsible for the presser effect of this peptide in rats; 4) LVV-H7 and FMRF amide-related peptides may share the same mechanism of presser activity in rats. (C) 1998 Elsevier Science Inc.