Eric F.C. Wong

Mechanism of Polyuria after Cisplatin Therapy

Cisplatin is an antineoplastic agent. Several nephrotoxic effects are associated with its use including chronic and acute renal failure, renal magnesium wasting, and polyuria. We have investigated polyuria in groups of rats treated with cisplatin at doses of 2.5 and 5 mg/kg body weight given once weekly for 3 weeks to determine possible mechanisms of this impairment. After cisplatin administration, glomerular filtration rate was reduced and significant increases in sodium and water loss were also seen. These changes were associated with decreases in urinary cAMP. Inner medullary collecting duct (IMCD) cells were removed from these animals and were stimulated with graded doses of vasopressin. Cells from cisplatin-treated rats showed an impaired response in cAMP generation to vasopressin stimulation as compared to cells from normal animals. To determine more precisely the site of impairment, the adenylate cyclase complex of the IMCD cells was further studied with forskolin and NaF. Forskolin was used to probe the catalytic unit activating adenylate cyclase, and NaF the guanine nucleotide regulatory protein (G protein). In response to forskolin, cells from cisplatin-treated rats and normal rats responded similarly in generating cAMP. However, following NaF, the cAMP response was blunted in the cells from the cisplatin rats. These results suggested that the catalytic unit was not injured by cisplatin (forskolin study) but the G protein was (NaF). In conclusion, the present study suggests that the polyuria seen following cisplatin administration is associated with an end-organ resistance to vasopressin manifested by reduced cAMP generation, secondary in part or whole to a defect at the level of the G protein.

Cyclic guanosine monophosphate responses to atrial natriuretic factor, brain natriuretic peptide, but not C-type natriuretic peptide, and the characterization of their receptors in rat medullary thick ascending limb

The effects of atrial natriuretic factor (ANF), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) on renal medullary thick ascending limb (mTAL) have not been fully understood. The aim of this study is to examine the second-messenger responses of rat mTAL to ANF, BNP, and CNP. Characterizations of the ANF, BNP, and CNP receptors in mTAL were also performed by radioligand studies. Results showed that ANF and BNP were both capable of eliciting cyclic guanosine monophosphate (cGMP) responses in mTAL. Conversely, no cGMP response was observed upon stimulation by CNP in mTAL. The presence of ANF receptors was demonstrated by radioligand studies. One receptor site was found, and the Kd and maximum binding capacity were 4.0 +/- 0.45 nmol/L and 277.8 +/- 47.7 fmol/mg protein, respectively. BNP receptors were also found in mTAL, and ANF and BNP were sharing the same receptor. On the contrary, no CNP receptor could be shown by radioligand studies. These results suggest that guanylyl cyclase-coupled receptors (atrial natriuretic peptide receptor-A [ANPR-A]) specific for ANF and BNP are present in rat mTAL, while those for CNP (ANPR-B) are absent. ANF and BNP but not CNP act on mTAL to control water excretion.

Changes in atrial natriuretic factor processing and secretion with age.

The effect of age on atrial natriuretic factor (ANF) metabolism in the rat was studied. Plasma ANF levels rose with age. A positive correlation was found between age and plasma ANF levels. Molecular forms of ANF were also examined. alpha-ANF and gamma-ANF were present in the circulation. The ratio of gamma-ANF/alpha-ANF increased with age. The atria were isolated and perfused to determine the ANF secretion rate. The spontaneous release of ANF decreased with age. The released ANF mainly consisted of alpha-ANF with a small amount of gamma-ANF. The ratio of gamma-ANF/alpha-ANF in the effluent increased with age. ANF concentrations in the atria varied with age, but the molecular forms did not. gamma-ANF was the main form found in the atria. These results suggest that ANF secretion as well as post-transcriptional processing is altered with age.

Hypersensitivity of inner medullary collecting duct cells to arginine vasopressin and forskolin in cardiomyopathic hamsters.

Circulating arginine vasopressin (AVP) levels are elevated in congestive heart failure (CHF). However, the second messenger changes of the AVP system in CHF have not been explored. The aim of the present study is to determine whether there are changes in cAMP production in the AVP system in CHF. Cardiomyopathic hamsters of strain UM-X7.1 were used. Normal Golden Syrian hamsters were used as controls. IMCD cells were isolated from both group of hamsters. cAMP accumulation experiments were performed with AVP and forskolin stimulation in vitro. There was more cAMP release after stimulation by both AVP and forskolin in the cardiomyopathic than normal hamster IMCD cells. This illustrates that hypersensitivity of IMCD cells to AVP exists in cardiomyopathic hamsters. This may be partly explained by the presence of V2 receptor adenylate cyclase hyperactivity. The hypersensitivity of IMCD cells to AVP may be one of the factors causing fluid retention in CHF.

Atrial natriuretic factor levels in renal stone patients with idiopathic hypercalciuria and in healthy controls: The effect of an oral calcium load

Ionized calcium is a stimulator for the release of several peptide hormones. Atrial natriuretic factor (ANF) is a peptide hormone released from atrial tissue in response to atrial distension or volume expansion. In the present study, we have examined the effect of an oral calcium load in healthy controls and renal stone patients with idiopathic hypercalciuria. Our results demonstrated that ANF release increased in both groups in response to a calcium load. However, idiopathic hypercalciuric patients presented lower basal ANF levels in the presence of high calcitriol levels. The role of calcitriol on ANF release remains to be evaluated.

In vitro Hormone-Stimulated Atrial Natriuretic Factor Release Is Increased in Experimental Renal Failure

The effects of hormone stimulation on atrial natriuretic factor (ANF) release in atria were studied in experimental renal failure rats. In vitro experiments were done in two groups of male Wistar rats. Group 1 rats were sham operated, and group 2 rats were subjected to 5/6 nephrectomy. Overall glomerular filtration rate was significantly reduced (1.98 +/- 0.10 vs. 0.75 +/- 0.05 ml/min, p < 0.001) in nephrectomized rats. These rats were also mildly uremic [blood urea nitrogen (BUN): 18 +/- 0.6 vs. 60 +/- 3.9 mg/dl p < 0.001]. The right atria of partially nephrectomized and sham-operated rats were isolated and perfused in a modified Langendorff apparatus to measure ANF release rate. Experiments were done in two phases. In the initial phase, spontaneous release of ANF was measured. In the second phase, angiotensin II (10(-6) M), vasopressin (10(-6) M) or endothelin (ET 1; 10(-6) M) were added into the perfusate. Spontaneous ANF release by the atria of renal failure rats was significantly elevated compared to intact rats. A significant positive correlation was found between ANF release rate and BUN (r = 0.65, p < 0.01). This suggests that the increase in ANF release by the atria of chronic renal failure (CRF) rats is related to the severity of renal impairment. Angiotensin II, vasopressin and endothelin induced exaggerated increases in ANF release by the atria of CRF rats. These results show that a shift in stimulus response curve is present and can contribute to the observed increase in plasma ANF levels in CRF rats.

The Differential Response in Atrial Natriuretic Peptide Release During Exercise in Patients with and Without Ischemic Heart Disease

ABSTRACT: The elevation of cardiac filling pressure induces the release of atrial natriuretic peptide into the circulation. Ischemia during exercise in patients with coronary artery disease may manifest itself with elevation of cardiac filling pressure before the onset of electrocardiographic changes or chest pain. Thus, patients with ischemic heart disease might have an elevated circulating atrial natriuretic peptide after exercise. The present study investigated the effect of exercise on circulating atrial natriuretic peptide in patients with and without ischemic heart diseases. Group 1 was composed of five patients who had ischemic heart disease by clinical history, previous myocardial infarction, angina or angiographically proven coronary artery disease and positive electrocardiogram during exercise. Group 2 was composed of five patients without ischemic heart disease and negative electrocardiogram response. Heart rate, blood pressure, and atrial natriuretic peptide were measured during routine treadmill exercise testing using the Bruce protocol. Our results indicate that the rate of rise of heart rate (12.3 ± 1.8 vs. 8.5 ± 0.7 beats/min/min), blood pressure (7.1 ± 1 vs. 4.2 ± 0.8 mm Hg/minute), and atrial natriuretic peptide (4.1 ± 1 vs. 1.4 ± 0.3 pg/ml/min) was significantly elevated in patients with ischemic heart disease compared to the group 2 patients. These findings suggest that the disporportionate elevation of atrial natriuretic peptide after exercise in ischemia may be caused by elevation of cardiac filling pressure, which may provide a nonivasive method for the diagnosis of ischemic heart disease.

Absence of C-Type Natriuretic Peptide Receptors in Hamster Glomeruli

The distribution of atrial natriuretic peptide receptor B (ANPR-B) varies between tissues and species. The aim of this study is to determine whether ANPR-B is present in the hamster glomeruli. In vitro C-type natriuretic peptide (CNP)- and atrial natriuretic factor (ANF)-stimulated cGMP accumulation studies were performed in hamster glomeruli. Elevated cGMP accumulations were observed upon ANF addition. No cGMP response was seen with CNP. Competitive receptor-binding experiments were performed with 125I-CNP and 125I-ANF against their respective cold peptides in hamster glomeruli. Although no CNP binding was detected, positive ANF binding was found and two types of ANF receptor were demonstrated. The affinity (Kdl) and maximum binding capacity (Bmaxl) of the high-affinity ANF receptor were 0.014 +/- 0.001 nM and 60.4 +/- 10.2 fmol/mg protein, respectively. Those of the low-affinity receptor (Kd2 and Bmax2) were 45.7 +/- 6.2 nM and 28.3 +/- 6.3 pmol/mg protein, respectively. Similarly, saturation binding experiments also failed to show any CNP receptor binding in hamster glomeruli. This finding suggests that ANPR-B is not present in hamster glomeruli and CNP is not a direct physiological regulator of hamster renal function.

Second Messenger Changes of Atrial Natriuretic Factor and Brain Natriuretic Peptide in Kidneys of Cardiomyopathic Hamsters

The effects of natriuretic peptides in kidney are blunted in congestive heart failure (CHF). The aim of this study is to examine the changes of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) second messenger productions in CHF. Experiments were conducted on 300-day-old normal and cardiomyopathic hamsters. Blood was collected for ANF measurement. cGMP accumulation studies were done in glomeruli upon ANF and BNP stimulation, and in inner medullary collecting duct (IMCD) cells upon ANF stimulation. Higher plasma ANF levels were found in cardiomyopathic hamsters (811.3 +/- 124.6 vs. 166.6 +/- 13 pg/ml, p < 0.01). ANF-stimulated cGMP accumulations in glomeruli and IMCD cells were higher in cardiomyopathic hamsters. Increased BNP-stimulated cGMP accumulations were also observed in cardiomyopathic hamster glomeruli. These results suggest that the renal hyporesponsiveness to natriuretic peptides in CHF in not due to attenuated ANF and BNP second messenger productions.

Effect of Dietary Sodium on Atrial Natriuretic Factor Released in Rats with Chronic Renal Failure

Studies were done in partially nephrectomized rats to examine the effect of dietary sodium intake on atrial natriuretic factor (ANF) released by the atria. Experiments were done in four groups of male Wistar rats. Group 1 (n = 10) and 3 (m = 10) rats were sham-operated. Group 2 and 4 were 5/6 nephrectomized. Group 1 and 2 were fed a sodium-supplemented diet. Group 3 and 4 received a sodium-deficient diet. Renal functions were similar between group 2 and 4. Plasma ANF level was raised in group 2 (182 +/- 17 pg/ml). Circulating ANF levels in group 1,3 and 4 were 95 +/- 5, 90 +/- 5 and 95 +/- 4 pg/ml, respectively. Atrial ANF contents were higher in partially nephrectomized rats after receiving a sodium-supplemented diet. A reduction in atrial ANF contents occurred when fed a sodium-deficient diet. In vitro studies were done to assess the rate of ANF released. ANF secretory rates were highest in group 2 (11 +/- 1.5 pg/min/mg). There was no difference between group 1,3 and 4. A positive correlation was found between plasma ANF and ANF released in all groups examined. Thus, plasma ANF levels were a good reflection of ANF secretory rates. A significant correlation existed between plasma ANF and sodium excretion in chronic renal failure rats (r = 0.78; p less than 0.01). A dissociation between plasma ANF and water excretion was seen. These results suggest that in chronic renal failure rats, ANF played a role in sodium adaptation.