Rudolf E. Lang

Blood levels and renal effects of atrial natriuretic peptide in normal man.

Since mammalian atria were recently found to contain vasoactive and natriuretic peptides, we investigated the following in normal humans: plasma human atrial natriuretic peptide concentrations, effective renal plasma flow (ERPF), glomerular filtration rate (GFR), urinary water and electrolyte excretion, blood pressure (BP), and catecholamine, antidiuretic hormone (ADH), angiotensin II, and aldosterone levels before, during, and after intravenous administration of the newly synthetized alpha-human atrial natriuretic peptide (alpha hANP). In 10 subjects alpha hANP given as an initial bolus of 50 micrograms followed by a 45-min maintenance infusion at 6.25 micrograms/min increased plasma alpha hANP from 58 +/- 12 to 625 +/- 87 (mean +/- SEM) pg/ml; caused an acute fall in diastolic BP (-12%, P less than 0.001) and a hemoconcentration (hematocrit +7%, P less than 0.01) not fully explained by a negative body fluid balance; increased GFR (+15%, P less than 0.05) despite unchanged or decreased ERPF (filtration fraction +37%, P less than 0.001); augmented (P less than 0.05- less than 0.001) urinary chloride (+317%), sodium (+224%), calcium (+158%), magnesium (+110%), phosphate excretion (+88%), and free water clearance (from -0.76 to +2.23 ml/min, P less than 0.001) with only little change in potassium excretion; and increased plasma norepinephrine (P less than 0.001) while plasma and urinary epinephrine and dopamine, and plasma ADH, angiotensin II, and aldosterone levels were unchanged. The magnitude and pattern of electrolyte and water excretion during alpha hANP infusion could not be accounted for by increased GFR alone. Therefore, in normal man, endogenous alpha hANP seems to circulate in blood. alpha hANP can cause a BP reduction and hemoconcentration which occur, at least in part, independently of diuresis and are accompanied by sympathetic activation. An increase in GFR that occurs in the presence of unchanged or even decreased total renal blood flow is an important but not sole mechanism of natriuresis and diuresis induced by alpha hANP in man.

ATRIAL NATRIURETIC PEPTIDE IN PLASMA OF VOLUME-OVERLOADED CHILDREN WITH CHRONIC RENAL FAILURE

The effect of volume changes on the plasma concentration of atrial natriuretic peptide (ANP) in children with chronic renal failure was investigated by the use of a specific and sensitive radioimmunoassay. Predialysis plasma ANP was significantly higher in children with end-stage renal disease than in healthy children and children with advanced renal failure without evidence of volume expansion. During haemodialysis plasma ANP decreased significantly and plasma renin activity rose slightly, whereas plasma arginine vasopressin and aldosterone did not change. Plasma ANP correlated positively with volume status (body weight gain during the interval between two haemodialysis sessions). An expanded extracellular fluid volume thus seems to be a major stimulus for the rise in ANP in children with end-stage renal disease. The findings suggest that ANP may be important in volume homoeostasis.

Evidence for the presence of enkephalins in the heart

Extracts of guinea pig hearts were subjected to high performance liquid chromatography (HPLC) and the eluted fractions monitored by radioimmunoassays (RIA) for their content of leucine5-enkephalin (Leu-ENK) and methionine5 enkephalin (Met-ENK). Distinct peaks of both Leu-ENK and Met-ENK immunoreactivity were found corresponding to the position of synthetic Leu-ENK and Met-ENK respectively. The ratio of Leu-ENK to Met-ENK content was about 1:4. Chemical sympathectomy with 5-hydroxydopamine (6-OH-DA) produced a dramatic fall in noradrenaline content of the heart by more than 99%, whereas the concentration in Leu-ENK was reduced by only 70%. The Leu-ENK content of the adrenal glands was not affected by this treatment. These observations point to an enkephalinergic innervation of the heart which appears to be mainly of sympathetic origin. The results suggest the participation of enkephalins in cardiac reflex mechanisms.

Persistent tissue converting enzyme inhibition following chronic treatment with Hoe498 and MK421 in spontaneously hypertensive rats.

Inhibition of angiotensin II generation in the plasma does not fully explain the antihypertensive effects of converting enzyme (CE) inhibitors. Thus, we investigated the role of CE inhibition in the tissue for the antihypertensive action of these drugs. After discontinuation of 4 weeks of oral treatment with either Hoe498 (3 mg/kg/day) or MK.421 (30 mg/kg/day) in spontaneously hypertensive rats (SHRSP), the reduced pressor response to intravenous angiotensin 1 was normalized within 1 day. although systolic and diastolic blood pressure remained decreased during a 1 week post-treatment follow-up period. Two weeks of oral treatment with Hoe498 (1 mg/kg day) and MK421 (30 mg/kg/day) in SHRSP lowered blood pressure markedly and inhibited CE in the plasma (43% and 22%). lung (85% and 33%). kidney (76% and 76%). aortic wall (75% and 48%). and (with Hoe498) in the heart (55%). After drug discontinuation, blood pressure remained decreased for an additional 2 weeks, whereas plasma CE was normal or elevated during the follow-up period. However, tissue CE activity measured 1 week after drug withdrawal was still inhibited in the aortic wall (67% and 30%) and in the kidney (48% and 41%). These results support the hypothesis that the prolonged antihypertensive action of CE inhibitors may be related to persistent CE inhibition in tissues such as vascular wall and kidney. Further, the data support the importance of CE inhibition at target sites other than plasma and lung vascular endothelium.

Is tissue converting enzyme inhibition a determinant of the antihypertensive efficacy of converting enzyme inhibitors? Studies with the two different compounds, Hoe498 and MK421, in spontaneously hypertensive rats.

The mechanism of antihypertensive action of converting enzyme (CE) inhibitors was studied by comparing two compounds of different potency. Spontaneously hypertensive rats (SHRSP) were treated orally with the CE inhibitors Hoe498 and MK421 for periods of 2 and 4 weeks. The threshold antihypertensive dose of Hoe498 was below 0.1 mg/kg/day, and of MK421 1 mg/ kg/day. Blood pressure was normalized with 10 mg/kg/ day Hoe498 and 30 mg/kg/day MK421. The reduction of the pressor responses to intravenous angiotensin I (ANG I), the potentiation of the bradykinin depressor responses, and the decreases of plasma angiotensin II levels or urinary aldosterone excretion were nor correlated to the antihypertensive potency of the drugs. However, following equi-dose treatment (10 mg/kg/day) with both compounds inhibition of tissue CE activity in the vascular wall of the aorta, in the lung, the kidney, and the brain cortex was more marked with Hoe498 than with MK421. Hoe498 also reduced CE activity in the heart, and adrenal gland, and in other brain areas, and attenuated the pressor responses to intracerebroventricularly injected ANG I. Our results provide the first direct evidence for tissue CE inhibition following chronic oral CE inhibitor treatment, and support the contention that inhibition of the enzyme in target organs such as vascular wall, kidney, heart, or brain may be involved in the antihypertensive action of these drugs.

Gene Expression of Natriuretic Peptide Receptors in Myocardial Cells

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones that serve to unload the heart through their effects on the kidney and vasculature. Whether the heart itself represents a site of action for these peptides is currently the subject of debate. Although functional studies indicate that ANP has some effects on isolated myocytes, several studies have been unable to detect binding of the hormone to these cells. The present study demonstrates that the genes for all three natriuretic peptide receptor (NPR) subtypes, NPR-A, NPR-B, and NPR-C, are expressed in the rat heart. For microlocalization of the receptor mRNAs in myocytes and nonmyocytic cells, a combination of cell isolation and reverse transcription-polymerase chain reaction (RT-PCR) was used. Cardiac myocytes were isolated by enzymatic dissociation of rat ventricular tissue, purified by density gradient centrifugation, and collected as single cells under microscopic control. Analysis by RT-PCR revealed the presence of transcripts for NPR-A as well as NPR-B and NPR-C. However, cGMP generation in purified myocytes was stimulated only by ANP and BNP, which specifically bind to NPR-A, whereas C-type natriuretic peptide (CNP, an NPR-B agonist) was ineffective. Therefore, rat ventricular myocytes appear to produce predominantly NPR-A. The expression of NPR-B may be low or even absent. The mRNAs for all three NPRs were also found in cultures of fibroblasts from the rat heart. In contrast to the myocytes, large increases in cGMP were observed in response not only to ANP but also to CNP.

Effects of lesions in the paraventricular nucleus of the hypothalamus on vasopressin and oxytocin contents in brainstem and spinal cord of rat

The effects of lesions of the paraventricular nucleus in rat hypothalamus (PVN) on the vasopressin (AVP) and oxytocin (OT) contents of the brainstem and spinal cord, as measured by radioimmunoassay, were studied. AVP decreased by 50% and 80% in brainstem and spinal cord of lesioned animals, whereas OT disappeared almost completely. Therefore, in contrast to OT, the PVN is not the only site of origin of AVP-containing nerve fibers projecting to the brainstem.

Endothelin has potent inotropic effects in rat atria

Endothelin, a novel peptide recently isolated from aortic endothelial cells, is one of the most active vasoconstrictors yet characterized. We report that endothelin is also a very potent inotropic substance, as demonstrated in isolated rat heart atria. The effects were observed at nanomolar concentrations and appeared to be due to a direct action of the peptide on the heart muscle cells. Thus, endothelin is not only a powerful vasoconstrictor but also has potent contractile effects on cardiac myocytes.

Brain angiotensin: on the way to becoming a well-studied neuropeptide system.

More than thirty different peptides in the central nervous system have been described in recent years. Some of these peptides are well defined with respect to their biosynthetic pathways, their distribution in the body, and their biological functions; other peptides have been merely identified by immunohistochemical methods (for review see Refs. l-3). Angiotensin II (ANG II) is one such newly discovered neuropeptide. Its biosynthetic pathway and functions in the periphery as a plasma hormone are well recognized and characterized (see Fig. 1). However, when components of the renin-angiotensin system (RAS) were first described in the brain [4,.5], the reports went largely unnoticed and were then met with scepticism [6). This lack of attention and the scepticism were perhaps related to the overwhelming attention given to the peripheral renin-angiotensin system, as well as to the little notice given to neuropeptides at the time. Currently, the brain RAS is on its way to becoming one of the best studied enzyme-neuropeptide systems. This subject has been reviewed recently [7] and only new advances will be discussed here.

Atrial natriuretic peptide secretion: synergistic effect of phorbol ester and A23187.

To investigate the role of inositol phospholipid turnover in the atrial natriuretic peptide (ANP) secretion, the secretory responses from isolated perfused rat hearts to the ionophore, A23187, and the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), alone or in combination, were studied. A23187 induced a sharp increase in ANP secretion, whereas TPA caused a slowly progressive increase in secretion rate. 4 alpha-phorbol-12,13-didecanoate, which lacks the ability to activate protein kinase C, had no effect on ANP secretion. The combination of A23187 and TPA stimulated ANP secretion higher than the calculated additive value for each agent. The synergistic effect of the agents suggests a role of calcium-activated protein kinase C in ANP secretion from atrial cardiocytes.