Noriyuki Kohashi

Mechanism of Taurine Natriuresis in Rats

Taurine, a sulfur containing amino acid, is abundant in many organs and tissues in mammals1. However, its physiological significance has been hardly understood. We reported that daily urinary taurine excretion significantly decreased in patients with essential hypertension, and administration of taurine increased urinary kallikrein and decreased blood pressure2. In the field of experimental hypertension animal, administration of taurine also showed antihypertensive effect in spontaneously hypertensive rats (SHR)3 and in a stroke-prone substrain (SHR-SP)4, in which the deficiency of taurine metabolism was demonstrated3,5. Since urinary kallikrein has a positive correlation with urinary kinin6, it seems that taurine stimulates kallikrein activity to increase the quantity of kinin produced in the kidney.

Regulation of aldosterone receptor in rat kidney cytosol by atrial natriuretic factor.

We investigated the effect of atrial natriuretic factor (ANF) on aldosterone receptors in the kidney cytosol, because the binding of aldosterone to aldosterone receptors in the cytosol is considered a critical step of its action. Rat atriopeptin III was injected into male Sprague-Dawley rats (200-250 g) via the femoral vein while under pentobarbital anesthesia, and aldosterone receptors in the kidney cytosol were determined. The maximum binding capacity and dissociation constant were calculated by the Scatchard analysis. Maximum binding capacity of both types of aldosterone receptor (Type I, high affinity and low binding capacity and Type II, low affinity and high binding capacity) gradually decreased after ANF injection, reached the lowest level after 2 hours, and then slightly recovered. When more than 2.5 micrograms/kg of rat atriopeptin III was injected, the density of aldosterone receptors significantly decreased. Injection of 12.5 micrograms/kg of rat atriopeptin III decreased maximum binding capacity of Type I receptor from 42.3 +/- 2.4 (mean +/- SD, n = 6) to 22.8 +/- 3.2 femtomole/mg protein (n = 6) (p less than 0.01), and that of Type II receptor decreased from 388 +/- 46 to 285 +/- 30 fmol/mg protein (p less than 0.01). Dissociation constant of both types of receptors did not change significantly after ANF injection. Plasma aldosterone concentration showed no significant change after ANF injection, and a significant change was noted after ANF administration on aldosterone receptors in the experiments on adrenalectomized rats 7 days after operation. Furosemide had no significant effect on aldosterone receptors in both normal and adrenalectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat platelets activate high molecular weight atrial natriuretic peptides in vitro.

Evidence suggests that the more biologically active low molecular weight forms (less than 10,000) of rat atrial natriuretic peptides are proteolytically derived from a less active precursor of higher molecular weight. Conversion and activation could occur within the myocyte as well as during circulation. The present study found that in vitro rat blood and platelets were capable of converting the high molecular weight atrial natriuretic peptides (greater than 10,000) to low molecular weight atrial natriuretic peptides within minutes and that enhanced biological activity attended the conversion. Rat high molecular weight peptides were partially purified by gel filtration, lyophilized, and reconstituted in Krebs-Ringer bicarbonate buffer. One milliliter of fresh rat blood was incubated with the high molecular weight peptides at 37 degrees C for 2 minutes. After centrifugation, the supernatant was fractionated on Sephadex G-75. Natriuretic activity was determined by bioassay in anesthetized rats. In contrast to the results following incubation of high molecular weight peptides in Krebs-Ringer bicarbonate buffer alone, which showed that 95% of the natriuretic activity remained in the high molecular weight peptide region, the natriuretic activity of the blood-treated high molecular weight peptides eluted almost exclusively in the low molecular weight peptide region, which indicates conversion. Blood was separated into plasma, erythrocytes, lymphocytes, and platelets. Conversion of high to low molecular weight peptides occurred only after incubation with platelets. Compared with control high molecular weight peptides, rat platelet-treated high molecular weight peptides had significantly greater activity in relaxing histamine-contracted rabbit aortic smooth muscle (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Rat atrial natriuretic peptides inhibit oxygen consumption by rat kidney.

The inhibitory effect of high and low molecular weight native and synthetic rat atrial peptides on oxygen consumption in isolated rat kidney mitochondria and slices was measured. Oxygen consumption by mitochondria was measured in the presence of succinate and/or adenosine diphosphate, furosemide, and low and high molecular weight native and synthetic rat atrial peptides. After the addition of succinate, adenosine diphosphate limiting respiration (State 4) increased in the presence of low, but not high, molecular weight native rat atrial peptides. Furosemide caused a significant decrease in State 4 respiration (p less than 0.001). Angiotensin II and arginine vasopressin did not alter State 4 respiration. The rate of oxygen consumption after the addition of saturating adenosine diphosphate in the presence of saturating succinate (State 3 respiration) was reduced by low and high molecular weight native rat atrial peptides. Furosemide completely blocked oxygen consumption after the addition of adenosine diphosphate. Oxygen consumption was unchanged by trypsin treated (natriuretically inactive) low molecular weight rat atrial peptides and ventricular protein extracts of high and low molecular weight native rat atrial peptides. Synthetic and low molecular weight native rat atrial peptides had similar effects on mitochondrial oxygen consumption. Low molecular weight native and synthetic rat atrial peptides decreased the adenosine diphosphate to oxygen ratio, and these peptides, as well as furosemide, also induced mitochondrial swelling; none of the other rat atrial peptide combinations nor angiotensin II produced this effect. In kidney slices, basal oxygen consumption (without substrates) was stimulated by succinate.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of Atrial Natriuretic Peptide by Kallikrein in Taurine Administered Rats

Taurine is a sulfur-containing amino acid abundant in many mammalian organs and reported to have some important roles1. However, physiological meaning is not fully understood. We have been investigating the antihypertensive effects of taurine, especially in relation to natriuresis2. The taurine natriuresis is supposed to be mainly concerned to the activation of the kallikrein-kinin system. This mechanism will be discussed in detail in another chapter in this review by our colleague, Dr. N. Kohashi. By the way, it became realized that atrial natriuretic peptide(ANP) may play some important roles in water-electrolyte balances and the potent natriuretic activity of ANP has been focused recently3,4, because ANP became to be able to be synthesized more easily, after amino acid sequence of ANP was determined5-8. So it may be increasingly important and neccessary to discuss the action of the natriuretic agents such as taurine in relation to ANP. The purpose of this paper is to examine how ANP takes part in the natriuresis induced by taurine and then assess the interaction of ANP and the activated kallikrein-kinin system induced by taurine. According to this purpose, we designed the following three experiments. 1 Effect of long term taurine administration on urinary kallikrein, atrial and plasma ANP. 2 Acute effect of ANP on natriuresis on rats with long term taurine administration. 3 Natriuresis of ANP in taurine administered rats with inhibiting kallikrein activity by aprotinin.