Eisaku Kitamura

Impaired contractile responsiveness of diabetic glomeruli to angiotensin II: a possible indication of mesangial dysfunction in diabetes mellitus.

The contractile function of renal glomerulus was studied in vitro using isolated glomeruli from streptozotocin-diabetic rats. Glomerular contraction was assessed by the reduction of extracellular [3H]inulin space of glomerulus, mostly composing of intracapillary space, produced by angiotensin II. The inulin space was dose-dependently reduced after angiotensin II addition in both diabetic and control rats but the degree of reduction significantly smaller in the former. The radioreceptor assay revealed rather increased angiotensin II receptors in diabetic glomeruli. Since the contractile response of glomerulus to angiotensin II is mediated via mesangial cell contraction, these results suggest the presence of mesangial cell dysfunction in diabetes.

Biphasic Alteration of Renin-Angiotensin-Aldosterone System in Streptozotocin-Diabetic Rats

The alteration of renin-angiotensin-aldosterone system caused by diabetes mellitus was studied in streptozotocin-diabetic rats. The plasma renin activity (PRA), and plasma levels of angiotensin II (A II) and aldosterone (PAC) were measured in diabetic and age-matched control rats in 1, 2, 4 and 8 weeks after the intravenous injection of streptozotocin (50 mg/kg body weight). Diabetic rats showed the marked hyperglycemia persistently throughout the experimental period. On 1st week PRA, A II and PAC were significantly increased, and A II and PAC were also significantly elevated on 2nd week in diabetic rats compared with control rats. However, on 4th and 8th weeks PRA, A II and PAC in diabetic rats were significantly lower than those of control rats. Hematocrit values in diabetic rats were elevated on 1st week, normalized on 2nd and 4th weeks and then decreased on 8th week. These results may suggest that the hyperglycemia causes a biphasic alteration of renin-angiotensin-aldosterone system, i.e., early stimulated state due to volume depletion and later suppressed state due to volume expansion.

Effect of angiotensin II infusion on glomerular angiotensin II receptor in rats.

Previous studies by other investigators have shown that sodium depletion causes down-regulation of angiotensin II receptors in renal glomeruli, which is ascribed to elevation of circulating angiotensin II levels. The present study was designed to determine whether elevation of circulating angiotensin II levels alone can down-regulate its own receptors without changes in the sodium balance. Rats were infused with angiotensin II at 50 ng/min intraperitoneally for 24 h or 7 d, then glomeruli were isolated by a mechanical sieving technique and used in a radioreceptor assay for angiotensin II. Angiotensin II infusion for 24 h and for 7 d both significantly suppressed plasma renin activity, and elevated plasma angiotensin II level (3-fold) but did not affect the plasma sodium and potassium or differences between dietary intake and urinary excretion of these electrolytes. By Scatchard analyses significant down-regulation of angiotensin II receptor number was demonstrated in renal glomeruli derived from rats infused with angiotensin II for either 24 h (32% decrease from the control value) or 7 d (37% decrease). No significant changes in receptor affinity were observed after 24 h of angiotensin II infusion, although the infusion for 7 d slightly but significantly decreased Kd (to 2.57 +/- 0.08 nM from 3.17 +/- 0.19, P less than 0.01). From these results, we conclude that circulating angiotensin II level itself can regulate the number of its own receptors in renal glomeruli even in the absence of changes in the sodium balance.

Effect of duration of diabetic state on insulin action in isolated rat soleus muscles.

We studied the effect of the duration of diabetic state on insulin action in skeletal muscle by measuring insulin binding, 2-deoxyglucose uptake, and intracellular glucose metabolism in isolated soleus muscles from streptozotocin-induced diabetic rats. Insulin binding to soleus muscles from diabetic rats was increased over that from controls. Glucose transport activity was determined by measuring the 2-deoxyglucose uptake at the concentration of 1 mmol/L at 25 degrees C. In the rats with diabetes of one week duration, insulin-stimulated 2-deoxyglucose uptake was not impaired, whereas basal 2-deoxyglucose uptake was decreased. However, the diabetic rats with two weeks duration revealed a 35.6% decrease in the insulin-stimulated 2-deoxyglucose uptake. Furthermore, four week duration of diabetic state led to a 60% decrease both in basal and insulin-stimulated 2-deoxyglucose uptake. Total glucose utilization was estimated as the total amount of glucose incorporated into muscle and lactate released into the medium following incubation at 37 degrees C, with 5 mmol/L glucose. The diabetic rats with one week duration did not demonstrate any changes in total glucose utilization both in basal and insulin-stimulated state. However more than two weeks duration of diabetes led to a 30% to 35% decrease both in basal and insulin-stimulated total glucose utilization, similar to the findings in the 2-deoxyglucose uptake study. We concluded that prolonged insulinopenia led to decreased glucose transport and intracellular glucose metabolism and resulted in insulin resistance in skeletal muscles.

Angiotensin II Effects upon Glomerular Intracapillary Volume in the Rat

The direct action of angiotensin II on renal glomeruli was quantified by measuring extracellular space in isolated, decapsulated rat glomeruli using 14C-inulin. The glomerular inulin space significantly decreased when treated with 0.1 nM to 1 microM of angiotensin II. The decrease was dose-dependent and blocked by 1-Sar, 8-Ile angiotensin II. Since the glomerular intracapillary space seems to occupy the majority of the extracellular (inulin) space in decapsulated glomeruli, these results suggest that angiotensin Ii decreases the glomerular intracapillary volume. This action of angiotensin II may lead to a reduction in glomerular filtering surface area, and thus, in the ultrafiltration coefficient.

Temperature-dependent negative cooperativity among angiotensin II receptors of isolated rat glomeruli

The binding characteristics of angiotensin II to isolated rat glomeruli were studied at various temperatures from 15 to 37 degrees C using 125I-labeled angiotensin II. The remarkable features of the binding at 37 degrees C, compared with those at lower temperatures, were (1) decreased maximal binding due to increased dissociation rate, (2) short duration of the steady state, which was, however, sufficient for performing steady-state binding assays, and (3) marked curvilinear Scatchard plots with upward concavity. The difference between the dissociation rates caused by dilution only and by dilution plus cold angiotensin II increased with increase in temperature. From these results, we conclude that the site-site interactions of a negatively cooperative type, which are negligible at lower temperatures, exist among rat glomerular angiotensin II receptors at the physiological temperature, and that the binding study may as well be performed at 37 degrees C for the purpose of investigating quantitative correlation between the hormone binding and its biological effect.

Short-Term Protein Load in Assessment of Guanidinoacetic Acid Synthesis in Patients with Chronic Renal Failure

Guanidinoacetic acid (GAA) is the precursor of creatine, an essential element.in the energy metabolism of muscle and nerve tissue. GAA has been demonstrated to be formed by the interaction of arginine and glycine mainly in the kidney.

Creatinine metabolism in diabetic nephropathy

糖尿病性腎不全の透析患者に占める割合は増加の一途を辿っているが, その予後は不良である.糖尿病性腎不全患者はその進行と共に, lean body massや筋力の低下, さらには全身臓器の機能低下が予想されるが, 今回これらの点に関し, 筋・神経組織のエネルギー代謝に必須なcreatine代謝の終末産物であるcreatinine (Cr) に注目し, Cr値が約2mg/dlの腎不全初期から透析に至る動態を検討した. その結果, 1) 糖尿病患者ではCr値が約2mg/dlの腎不全初期から透析導入までの期間は平均約2年と短いにもかかわらず, 体重は著明に減少し, 腎不全の経過中急速にlean body massが減少していた. これは経過中の尿中Cr排泄量の減少によっても裏づけられた. 2) 透析導入直前の尿中Cr排泄量は, Cockcroftらの考案した年齢, 性別, 透析除水後の体重から算出した予測値, さらには慢性腎炎患者の尿中Cr排泄量に比べても有意に低値であり, 除水後の体重設定にも問題はあるが, 糖尿病腎不全患者では特にlean body mass当りのCr産生の低下, すなわち筋肉・神経組織内のcreatineリン酸含量の低下が推測される. これはcreatineリン酸, ひいてはCrの前駆物質であり, 腎で産生されるguanidinoacetic acidの減少によるものと考えられ, 尿毒症病態の一因と成っている可能性がある. 3) 臨床的にも糖尿病患者の管理上, 定期的な尿中Cr排泄量の測定が, 正確な病態や腎機能の把握に有用な指標となりうる.