Takushi X. Watanabe

Cellular mechanism of action by a novel vasoconstrictor endothelin in cultured rat vascular smooth muscle cells

Specific binding sites for synthetic porcine endothelin (pET), a novel potent vasoconstrictor peptide isolated from the supernatant of cultured porcine endothelial cells, and its effects on cytosolic free Ca2+ concentrations ([Ca2+]i) and phosphatidylinositol (PI) response were studied in cultured rat aortic vascular smooth muscle cells (VSMC). Binding of 125I-labeled-pET to rat VSMC was time- and temperature-dependent and the cell-bound 125I-labeled-pET was resistant to dissociate. Scatchard analysis of binding studies indicated the presence of a single class of high-affinity binding sites: the apparent Kd was 2-4 X 10(-10) M and the maximal binding capacity was 11,000-13,000 sites/cell. The binding was highly specific for pET because neither well-recognized vasoconstrictors, peptide neurotoxins, nor Ca2+-channel blockers affected the binding. pET dose-dependently (10(-9)-10(-7) M) induced a transient and sustained increase in [Ca2+]i in fura-2-loaded cells of which effect was largely dependent on extracellular Ca2+, whereas it had no significant effect on PI response in 3H-myoinositol-prelabeled cells. The present data clearly demonstrates the presence of specific receptors for pET distinct from those of the well-recognized vasoconstrictors and voltage-dependent Ca2+-channels in cultured rat VSMC, and suggest that pET-induced increase in [Ca2+]i is involved in the mechanism of its vasoconstriction.

Specific receptors for adrenomedullin in cultured rat vascular smooth muscle cells

The effects of synthetic rat adrenomedullin (rAM), a novel vasorelaxant peptide originally isolated from human pheochromocytoma, on receptor binding and cAMP generation were studied in cultured rat vascular smooth muscle cells (VSMC). A binding study using [125I]rAM revealed the presence of a single class of high‐affinity (K d1.3 × 10−8 M) binding sites for rAM in VSMC. The apparent K i of rat calcitonin gene‐related peptide (rCGRP) was 3 × 10−7 M. Affinity labeling of VSMC membranes with [125I]rAM revealed two distinct labeled bands with apparent molecular weights of 120 and 70 kDa, both of which were abolished by excess unlabeled rAM or rCGRP. rAM stimulated cAMP formation with an approximate EC50 of 10−8 M, the effect of which was additive with isoproterenol, but not with rCGRP. The rAM‐induced cAMP response was unaffected by propranalol, indomethacin, or quinaerine, but inhibited by a CGRP receptor antagonist, human CGRP[8–37]. These data suggest that VSMC possesses specific AM receptors functionally coupled to adenylate cyclase with which CGRP interacts.

Structure-activity relationship of endothelin: Importance of charged groups

Endothelin (ET)-related peptides including ET-1 (1-39) were synthesized, and their constricting activity in rat pulmonary artery rings and pressor activity in unanesthetized rat were measured to elucidate their structure-activity relationship. The vasoconstrictor activities of ET-2, ET-3 and sarafotoxin S6b were one-half, one-60th and one-third that of ET-1, respectively. Such differences in biological activities should mainly arise from sequence heterogeneity at the N-terminal portion, especially at positions 4 to 7. All of the blocked ETs at the amino or carboxyl termini showed greatly decreased activities. A monocyclic analog, in which Cys3 and Cys11 were replaced by Ala, showed one-third the activity of ET-1; however, its deamino dicarba analog was almost completely inactive. Significant activities were retained even with replacement of amino acids at positions Ser4, Ser5, Leu6, Met7, Lys9, Tyr13, and Trp21 by Ala, Ala, Gly, Met(0), Leu, Phe, and Tyr or Phe, respectively. On the other hand, replacement of Asp8, Glu10 and Phe14 by Asn, Gln and Ala, respectively, resulted in complete loss of the biological activity. These results indicated that two disulfide bonds in ET molecule were not essential for the expression of vasoconstricting activity. Both terminal amino and carboxyl groups, carboxyl groups of Asp8 and Glu10, and the aromatic group of Phe14 seemed to be contributing, more or less, to the expression of the biological activities.

Amino acid sequence and relative biological activity of eel atrial natriuretic peptide

A peptide exhibiting vasodepressor and natriuretic activities in rats was isolated from eel atria, and its primary structure was determined as H-Ser-Lys-Ser-Ser-Ser-Pro-Cys-Phe-Gly-Gly-Lys-Leu-Asp-Arg-Ile-Gly-Ser-Tyr-Ser- Gly-Leu-Gly-Cys-Asn-Ser-Arg-Lys-OH. This peptide, termed eel atrial natriuretic peptide (ANP), has sequence homology of 59% to mammalian (human or rat) ANP, 52% to fowl ANP, and 46% to frog ANP. When the biological activity of synthetic eel ANP was compared with that of human ANP, the eel peptide was 110 times more potent for the vasodepressor activity in eels, nearly equipotent for the vasodepressor activity in quails, and 20 times less potent for the vasodepressor and natriuretic activity in rats.

A novel natriuretic peptide isolated from eel cardiac ventricles

A new natriuretic peptide, which exhibits the entire spectrum of actions known to be characteristics of atrial and brain natriuretic peptides (ANP and BNP), was isolated from eel cardiac ventricles and has been named ventricular natriuretic peptide (VNP). The primary structure of eel VNP is characterized by its uniquely long C‐ terminal ‘tall’ that extends from the second half‐cystine. Thus, eel VNP appears to be a novel natriuretic peptide of a type not found in mammals. With respect to natriuretic (rat) and vasodepressor (rat and eel) activities, eel VNP is much more potent than human ANP in eels and almost equipotent in rats. Strong tachyphylaxis is observed for the vasodepressor effect in both rats and eels, whereas it is not observed for the natriuretic effect in rats.

Synthesis of endothelin-1 analogues, endothelin-3, and sarafotoxin S6b: Structure - activity relationships

Summary Two disulfide analogues (types A and B) of endothelin-3 (ET-3; formerly, rat ET), sarafotoxin S6b, and apamin, were synthesized to determine their disulfide structures as in the case of endothelin-1 (ET-1; formerly human and porcine ET). The disulfide structures of ET-3 and sarafotoxin S6b were found to be identical with that of ET-1 (type A) but distinct from that of apamin (type B). The vasoconstricting activities of ET-3 and sarafotoxin S6b were about one-60th and one-third that of ET-1, respectively. Such different biological potencies between endothelins and sarafotoxin S6b could be largely attributed to the sequence heterogeneity at the N-terminal portion. ET-1 analogues were also synthesized to clarify the structure-activity relationships. The opening of any disulfide bond in the ET-1 molecule extremely decreased the activity, while oxidation of the Met residue did not alter it. Amidation of the terminal COOH group and extension of the Lys-Arg sequence to the N-terminus led to 16-and 540-fold decreases in activity, respectively. Removal of the C-terminal Trp residue resulted in complete loss of the activity. The other disulfide analogues (type B and C) of ET-1 showed markedly lower activity than the parent molecule (type A). These results indicated the importance of the whole molecule with the proper double cyclic structure for determining its active conformation.

Receptor binding activity and cytosolic free calcium response by synthetic endothelin analogs in cultured rat vascular smooth muscle cells

Using a variety of synthetic analogs of porcine endothelin (pET), we have studied the effects of these analogs on receptor binding activity and cytosolic free Ca2+ concentrations ([Ca2+]i) in cultured rat vascular smooth muscle cells (VSMC). Removal of C-terminal Trp21 residue, truncated derivatives pET(1-15) and (16-21), substitution of disulfide bond, Cys(3-11) or Cys(1-15), by Cys (Acm), all resulted in a complete loss of receptor binding activity and [Ca2+]i response, while N-terminal elongation of Lys-Arg residues, but not oxidation of Met7 residue, decreased receptor binding activity and [Ca2+]i response. [Cys1-15,Cys3-11]pET was far more potent than [Cys1-11,Cys3-15]pET in receptor binding and [Ca2+]i response. These data indicate that the C-terminal Trp21 as well as the proper double cyclic structure formed by the intramolecular disulfide bonds of the pET molecule are essential for receptor binding and subsequent [Ca2+]i increase in rat VSMC.

Chemical structure of angiotensin in the bullfrog Rana catesbeiana

Crude bullfrog angiotensin was prepared by modified Bouchers procedures. The kidney extract was incubated in vitro with homologous plasma. Crude angiotension, 15 micrograms equivalent to [Asp1, Ile5]angiotensin II in rat vasopressor assay, was purified further by three steps by SEP-PAK C18 cartridge, SP-Sephadex column chromatography, and high-performance liquid chromatography on Finepak SIL C18 column. Amino acid analysis was done on 64 ng of the purified material after acid hydrolysis. The fluorescent peptide mapping technique was used to confirm the amino acid sequence. It is proposed that bullfrog angiotensin is [Asp1, Val5, Asn9]angiotensin I.

Chemical structures of angiotensins formed by incubating plasma with the kidney and the corpuscles of Stannius in the chum salmon, Oncorhynchus keta

The chemical structures of salmon angiotensins produced by incubating tissue extract of the kidney or the corpuscles of Stannius (CS) with homologous plasma are proposed. Two angiotensins, [Asp1, Val5, Asn9] and [Asn1, Val5, Asn9] angiotensin I, were proposed from both kidney and CS incubations by amino acid analysis and the fluorescent peptide-mapping techniques. CS angiotensins were not organ specific, because these two angiotensins were produced by both kidney and CS incubations in a ratio of 1:2 under the same conditions. Whether [Asp1, Val5, Asn9] angiotensin I is a naturally occurring form remains to be clarified; however, [Asn1, Val5, Asn9] angiotensin I may be the major form of angiotensin formed from plasma by salmon kidney and CS.

Receptor binding and vasoconstrictor activity of big endothelin

We studied the effects of synthetic porcine big endothelin (ET), a putative 39-residue intermediate deduced from cDNA sequence analysis, on receptor binding activity in cultured rat vascular smooth muscle cells as well as its vasoconstrictive activity in rat pulmonary artery. Big ET was about two orders of magnitude less active than ET in the receptor binding and vasoconstriction assays. Our data are consistent with the contention that big ET is an intermediate which can be converted into fully bioactive 21-residue mature ET.