Effect of short- and long-term stress on plasma calcium and calcitonin in the rat

Although hypocalcemia has been observed during stress, the cause(s) is still unknown. The effect of short- and long-term restraint stress on calcitonin (CT) secretion and on plasma calcium (Ca), inorganic phosphate (P), total protein (TP), and corticosterone (CORT) were investigated in the male and female Sprague-Dawley rat. The rats were restrained in the supine position for two hours/day for either one day (STS) or for 14 days (LTS), and compared to normal controls (CON). Plasma Ca levels in both STS and LTS rats were significantly lower and mean plasma CORT levels in female stressed rats were significantly higher than those in sex-matched CON rats. However, mean basal levels of plasma CT did not differ among these three groups for either sex. Similarly, mean increment of plasma CT after Ca infusion (Ca 10 mg/100 g, i.v.) did not differ between STS or LTS rats and CON rats. These data reveal no causal relationship between CT and the hypocalcemia during either short-or long-term stressful stimuli.     

Effect of electrical stimulation of the vagus on plasma motilin concentration in dog

Ten dogs anesthetized with α-chloralose were prepared with platinum monopolar electrodes in the antrum, duodenum and jejunum to record myoelectrical activity and bipolar stimulating electrodes placed on distal cut end of both cervical vagi to apply electric stimulation. Blood samples were obtained from both portal and femoral veins before and after bilateral vagal stimulation was initiated while the myoelectric activity was recorded continuously. The stimulation parameters used were low frequency (9V, 5 cps, 0.5 ms) and high frequency stimulus (9V, 30 cps, 10 ms) for 10 min. During the stimulation, plasma motilin concentrations increased significantly in both portal and femoral veins with simultaneous increases in the spike activity. The increment in the motilin level of portal venous blood was more marked. In 7 dogs, high frequency stimulation was repeated while the animals received i.v. atropine, 100 μg/kg-hr. Atropinization completely blocked the increase in the motilin concentration in response to high frequency stimulus with a simultaneous inhibition of the spike activity. The study suggests strongly that the vagus nerve plays an important role on endogenous release of motilin through its cholinergic pathway.     

The relationship between plasma concentration and disappearance rate of immunoreactive insulin in the conscious dog

The relationship between plasma concentration and disappearance (infusion) rate of insulin was determined during infusion of insulin via the portal circulation of 14 normal euglycaemic dogs when somatostatin was infused to block endogenous insulin secretion. The relationship could be represented by one straight line over the plasma immunoreactive insulin range 0 to 110 microU/ml (r = 0.99) but above 110 microU/ml the fractional insulin disappearance rate declined. The results indicate the existence in the dog of a saturable pathway of insulin degradation that could conceivably be located in liver and which may become saturated only at insulin concentrations in the portal vein exceeding approximately 330 microU/ml.     

Effect of calcitonin on fructose 1,6-diphosphatase activity in rat liver: role of cytosolic calcium concentration

The effect of calcitonin (CT) on fructose 1,6-diphosphatase activity in the hepatic cytosol was investigated after a single subcutaneous administration of the hormone to rats. Administration of CT (synthetic [Asu1,7] eel CT; 80 MRC mU/100 g body weight) produced significant increase in fructose 1,6-diphosphatase activity and calcium content in the hepatic cytosol of intact and thyroparathyroidectomized rats. Those alterations were also observed with the dose of CT at physiological level. The binding of calcium by 10 microM EGTA in the hepatic cytosol caused a clear reduction of the increase in fructose 1,6-diphosphatase activity produced by CT administration. The enzyme activity of CT-treated rats was markedly reduced by W-7 (100 microM), calmodulin inhibitor, while that of control rats was not significantly altered by the drug. Meanwhile, fructose 1,6-diphosphatase activity in the hepatic cytosol obtained from normal rats was significantly enhanced by addition of calcium ion (0.1-5.0 microM). This increase was also clearly inhibited by W-7. These results suggest that CT increases fructose 1,6-diphosphatase activity in the hepatic cytosol of rats, and that this hormonal regulation may depend on calmodulin, mediated through calcium increased in the cytosol.     

Testosterone metabolic clearance rate and production rate in the male infant rhesus monkey

The male infant rhesus monkey (Macaca mulatta) undergoes a period of testicular activation similar to that seen in the human infant. Plasma testosterone (T) concentrations rise after birth, reaching levels of about 500 ng/dl at 1-3 mo of age and then fall to approximately 50 ng/dl at 60 mo. The plasma T metabolic clearance rates (MCRT) and production rates (PRT) were measured in two rhesus infants at 1 and 6 mo of age to determine the mechanism of the observed increase in plasma T. While there was little change in the MCRT between 1 and 6 mo, PRT was much higher at 1 mo than at 60 mo of age. These observations are consistent with the hypothesis that the increased plasma testosterone levels in infant rhesus monkeys reflect an increased production of testosterone rather than an altered metabolic disposition of the hormone.     

Effects of salmon calcitonin and anti-salmon calcitonin antibody on plasma calcium concentrations in the goldfish,Carassius auratus

The role of calcitonin (CT) in plasma calcium regulation was studied by the administration of exogenous CT and anti-salmon(s) CT antibody using goldfish,Carasius auratus, loaded or otherwise with calcium. CT elicited a decrease in plasma calcium concentrations at a dose of 10 ng/g body weight 1 h after administration. However, no effects were observed following doses of 30 ng and 50 ng/g 1 h, nor for the three doses 3 h after administration. In calcium-loaded fish, the effect of CT was different depending on the dosage of CT. Ten ng and 50 ng/g induced a decrease and an increase in plasma calcium concentrations, respectively, 3 h after administration. Anti-sCT antibody (0.02 μg or 0.1 μg/g) did not affect plasma calcium concentrations. In calcium-loaded fish, neither dose of anti-sCT antibody changed plasma calcium concentrations 1 h after administration. However, following a dose of 0.1 μg/g, plasma calcium concentrations decreased after 3 h. A positive correlation between plasma calcium concentrations and the gonad somatic index (GSI) in females was no longer apparent after administration of anti-sCT antibody. There was no relationship between plasma calcium concentrations and GSI in control and anti-sCT antibody-treated males. These results suggested that CT regulates plasma calcium concentrations in different ways depending on the dosage with CT having a role in calcium physiology during vitellogenesis.     

The effect of porcine calcitonin on plasma calcium in Channa punctatm Bloch

The effect of porcine calcitonin on the plasma calcium of the freshwater Indian murrel, Channa punctatus Bloch has been studied. An injection of 200 MRC mU of porcine calcitonin was given intraperitoneally, while control fish received the gelatin carrier. A significant hypocalcaemia was noted I h after calcitonin treatment (P < 0.001). The plasma calcium had returned to the normal level by the 6th h.     

A dual effect of calcitonin gene-related peptide on plasma calcium levels in the chick

Calcitonin gene-related peptide (CGRP) lowers plasma calcium in the rat and inhibits bone resorption by isolated rat osteoclasts. In our preliminary studies we found that rat CGRP elevates plasma calcium levels in the chick, a response that was somewhat similar to that of parathyroid hormone. Here, we report that human CGRP (alpha) produces a concentration-dependent elevation of plasma calcium levels. The two peptides did not follow precisely the same time course. Whereas at 15 minutes CGRP produced hypocalcaemia relative to the control plasma calcium levels, at 30 minutes both CGRP and PTH were found to be hypercalcaemic. These studies suggest that CGRP initially interacts with the calcitonin receptor to produce a calcitonin-like effect, which is followed by hypercalcaemia presumably by antagonising the action of endogenous circulating calcitonin.     

Direct effect of plasma sex hormone binding globulin (SHBG) on the metabolic clearance rate of 17 beta-estradiol in the primate

Sex hormone binding globulin (SHBG) has been shown to be a major determinant of testosterone clearance in the primate. It has also been suggested that SHBG would also be a determinant of estradiol clearance (MCR-E2). However, published studies have suggested that the MCR-E2 do not always vary with changes in the level of SHBG. Therefore, the present study was undertaken to address this issue. The baseline MCR-E2 was determined in adult male pigtail macaques (Macaca nemestrina). Following the baseline determination of MCR-E2 the animals were infused with either purified human (h) SHBG or antibody against hSHBG, which also has a high degree of cross-reactivity with primate SHBG. Following the infusions of either hSHBG or anti-SHBG, MCR-E2 was again determined. In addition, luteinizing hormone (LH) was measured using a mouse Leydig cell bioassay. Following the infusion of hSHBG, a marked increase in serum SHBG was noted and the MCR-E2 decreased. Associated with the increase in SHBG, the SHBG bound T levels decreased and LH increased. Following the infusion of antibody, serum SHBG decreased, and the MCR-E2 also decreased. With the decrease in SHBG following the antibody infusion, non-SHBG bound T increased and serum LH fell. This study demonstrates that an increase in the serum SHBG levels is associated with a decrease in MCR-E2, however, an acute decrease in serum SHBG also decreases the MCR-E2. This later result demonstrates that factors in addition to serum SHBG binding may be important in determining the MCR-E2.     

Effect of a sudden and a slow concentration increase in plasma urea on its concentration in some tissues of the dog and rat

The urea concentration in the renal cortex ([RC]) of pentobarbital-anaesthetized dogs was 5.71 times higher than the plasma concentration ([P]); the liver ([L]) and the skeletal muscle ([SM]) concentrations were the same as ([P]). Rapid infusion of 20% urea (1 g/kg b.w. within 1 min) was followed by a sudden increase in [P]; [RC] and [L] rose to values nonsignificantly different from [P] and remained non-significantly different for the whole 4 hours of the experiment; at the end, [P] was still about 10 times higher than before infusion. Surprisingly, [SM] 2 and 6 min after infusion was significantly lower than [P]; later they were the same. The experiment thus does not testify to the existence of active transport of urea in the RC. The permeability of the skeletal muscle membrane for urea is lower than that of the RC and liver. Chronic uraemia was induced in rats by transplanting the trigonum vesicae into the peritoneal cavity. In addition to the chemical determination of urea, 14C-urea (marked*) was also measured. [RC]/[P] and [RC*]/[P*] fell as [P] rose; [L], [L*] [SM] and [SM*] never differed from [P] or [P*]. Fluid [PF] collected in the peritoneal cavity had the same chemically determined and radioactive urea concentration as P, but it was hypoosmolar and had a lower [Na+] than P. These experiments likewise did not testify to active urea transport in the RC. It is not clear what caused the osmolarity and sodium gradient between the PF and P, but the peritoneal wall definitely did not act as a simple dialyzing membrane.     

Comparative metabolic clearance rate, volume of distribution and plasma half-life of human beta-lipotropin and ACTH.

An antiserum to β_h-lipotropin (LPH) which does not cross react with β_h-endorphin has been obtained utilizing two different methods of affinity chromatography. This was employed in studies of three normal human subjects in whom the metabolic clearance rate (MCR) apparent volume of distribution (V_d) and fractional rate of disappearance (K_d) of ACTH and β_h-LPH were determined following bolus simultaneous injection of 270 μg highly purified β_h-LPH and 230 μg of synthetic human ACTH. A biphasic disappearance curve was noted for both hormones. $\text{β}{}_{\mathrm{h}}\text{-LPH}$: MCR-0.571, 0.519, and 0.461 L/minute; V_d-30.7, 27.7, 25.0 liters, representing 49, 46 and 35% of body weight; K_d-0.0186, 0.0187, 0.0185 min^?1. $\text{ACTH}$: MCR-0.274, 0.266, and 0.332 L/minute; V_d-6.5, 6.5, 14.5 liters, representing 10.4, 10.8 and 20.4% of body weight; K_d-0.0418, 0.0409, 0.0229 min^?1. The observed larger MCR of β_h-LPH can account for previous observations of basal plasma ACTH/LPH ratios greater than unity, even though these peptides are present in the pituitary in equimolar concentrations.     

The effects of progesterone supplementation on the metabolic clearance rate of progesterone in the pregnant rat

In the pregnant rat, short-term stability of progesterone blood concentrations may involve an active homeostatic mechanism. In the present study, we examined the possibility that the metabolic clearance rate (MCR) of progesterone can respond to a change in progesterone production and thus reduce variation in blood concentrations. Progesterone was administered both acutely and chronically to conscious rats, and the effective production rate, MCR, and blood concentration were monitored on Day 16 of pregnancy. Acute, low-dose progesterone supplementation, which effectively raised production rate by 29%, had no effect on the MCR of progesterone. Acute, high-dose supplementation, which raised total progesterone production by 68%, caused a 35% fall in the MCR of progesterone. Chronically supplemented rats received s.c. injections of progesterone (20 mg) once daily over Days 13-16 of pregnancy. The resultant production rate measured on Day 16 was 114% higher than that in controls, but there was no difference in MCR. Collectively, these experiments demonstrate that no short-term homeostatic mechanism involving the MCR operates to control blood progesterone concentrations in Day 16 pregnant rats. Thus, progesterone homeostasis appears limited to long-term developmental changes rather than short-term physiological control.