Impairment of renal sodium excretion in tropical residents – phenomenological analysis

There is evidence of impaired renal sodium excretion in salt-sensitive African Blacks. A decreased rate of renal sodium chloride (NaCl) excretion, low plasma renin activity and a tendency to elevated blood pressure are the hallmarks of salt sensitivity. Recent evidence indicates that increased proximal and distal tubular fluid reabsorption in some tropical residents may explain the impaired sodium excretion in these people. In this study of a cohort population, we speculated that subjects selected from that population might be salt-sensitive. We therefore measured the sodium balance in 10 normotensive male subjects over 10 consecutive days, after they had ingested a normal or a high amount of sodium, as NaCl (salt) in their diet. We quantified their renal sodium excretion rate by phenomenological analysis of their sodium balance data. We also measured plasma renin activity for 7 consecutive days in a separate group of 6 male and 4 female subjects in order to assess the state of their renin/angiotensin system. We selected all our subjects from a cohort population of 269 subjects randomly selected from a community known to have a high prevalence of primary hypertension. Our data on two separate groups of subjects from the same cohort population revealed delayed renal sodium excretion with t _1/2 of about 5 days, compared to published data for normal individuals with t _1/2 of less than 24 h. Also, plasma renin activity levels were low. Hence, our subjects are salt-sensitive. Quantification of their renal impairment is important for various reasons: it heightens one’s appreciation of the problem of salt retention in African Blacks who are salt-sensitive and it also underlines the importance of the need for further research into the benefits of dietary salt restriction for reducing cardiovascular mortality in African populations, as has been done in some Western countries. Received: 4 March 1999 / Accepted: 12 May 1999     

Increased renal tubular sodium reabsorption during exercise-induced hypervolemia in humans.

We tested the hypothesis that renal tubular Na(+) reabsorption increased during the first 24 h of exercise-induced plasma volume expansion. Renal function was assessed 1 day after no-exercise control (C) or intermittent cycle ergometer exercise (Ex, 85% of peak O(2) uptake) for 2 h before and 3 h after saline loading (12.5 ml/kg over 30 min) in seven subjects. Ex reduced renal blood flow (p-aminohippurate clearance) compared with C (0.83 +/- 0.12 vs. 1.49 +/- 0.24 l/min, P < 0.05) but did not influence glomerular filtration rates (97 +/- 10 ml/min, inulin clearance). Fractional tubular reabsorption of Na(+) in the proximal tubules was higher in Ex than in C (P < 0.05). Saline loading decreased fractional tubular reabsorption of Na(+) from 99.1 +/- 0.1 to 98.7 +/- 0.1% (P < 0.05) in C but not in Ex (99.3 +/- 0.1 to 99.4 +/- 0.1%). Saline loading reduced plasma renin activity and plasma arginine vasopressin levels in C and Ex, although the magnitude of decrease was greater in C (P < 0.05). These results indicate that, during the acute phase of exercise-induced plasma volume expansion, increased tubular Na(+) reabsorption is directed primarily to the proximal tubules and is associated with a decrease in renal blood flow. In addition, saline infusion caused a smaller reduction in fluid-regulating hormones in Ex. The attenuated volume-regulatory response acts to preserve distal tubular Na(+) reabsorption during saline infusion 24 h after exercise.     

Comparison of the natriuresis and chlorures is associated with glomerular hyperfiltration induced by atrial natriuretic factor or glucagon

The impact on renal sodium chloride reabsorption of an acute increase in glomerular filtration rate (GFR) induced by atrial natriuretic factor (ANF) or glucagon was examined in the conscious rat. These hormones have no direct effect on proximal solute transport and have opposite effects on distal transport. ANF and glucagon increased GFR to a comparable extent (2.0 ± 0.2 to 3.5 ± 0.4 ml/min, p<0.01, and 1.9 ± 0.1 to 3.3 ± 0.1 ml/min, p<0.001, respectively). While most (95–97%) of the increment in filtered sodium chloride was reabsorbed, a small portion (3–5%) escaped tubular reabsorption. Absolute sodium and chloride urinary excretion rates increased similarly in response to each hormone, by two- to three-fold. Slightly imperfect load-dependent sodium chloride reabsorptive response by the nephron, despite opposite direct effects on distal nephron transport, may account for the observed natriuresis and chloruresis associated with the acute glomerular hyperfiltration induced by ANF or glucagon administration.     

Effect of polyaspartic acid on CdCl2-induced nephrotoxicity in the rat

We produced an animal model of CdCl₂ nephrotoxicity in rats, and treated them with polyaspartic acid (PAA) to prevent renal damage. Male Sprague-Dawley (SD) rats (190–200 g) were used to induce proximal renal tubular damage by daily injection of CdCl₂ 3.0 mg/1,000 g body wt for 2 wk. CdCl₂-exposed SD rats exhibited significant increases in urine volume, urinary excretion ofN-acetyl-β-D-glucosaminidase (NAG), alanine aminopeptidase (AAP), and fractional excretion of sodium (FENa) and a decrease in the percentage of tubular reabsorption of phosphate (%TRP). Of these indicators of proximal tubular function, AAP and %TRP are more sensitive than NAG or FENa. No glycosuria or aminoaciduria, however, were observed. PAA markedly improved these indicators of proximal tubular function. Daily urinary protein excretion and creatinine clearance, on the other hand, did not change after administration of PAA. Cd concentrations in the cortex were 3 times higher than in the medulla, however, there were no differences between Cd-treated rats and PAA-treated rats. Our animal model is an excellent one for determining the effect of cadmium on renal proximal tubule damage. PAA appears to be useful in the treatment of CdCl₂ nephrotoxicity.     

Gonadal development and steroid hormone profile of wild caught grey mullet (Mugil cephalus)

The grey mullet Mugil cephalus is one of the popular and fast growing fishes being cultured in tropical and subtropical regions. In the present study, histological observation of gonadal development and corresponding changes in sex steroid levels from different maturity stages of wild caught male and female were studied. In female, testosterone and 17β-estradiol increased with the advancement of maturation and reached peak (17β-estradiol, 323 ± 13 pg/ml; testosterone, 938 ± 7.87 pg/ml) in mature stage, whereas the level of progesterone was maximum (488 ± 4.9 pg/ml) during ripe stage. Vitellogenin level in serum showed a similar trend as 17β-estradiol. In case of male, the testosterone level in serum increased gradually with advancement of maturation and was maximum (1820 ± 40.25 pg/ml) during ripe stage, whereas significant decrease in 17β-estradiol and progesterone was noticed with advancement of maturation. The fundamental information from this investigation would be useful for developing protocol for accelerating maturation and spawning under captive condition.     

Chronopharmacology of Methotrexate Pharmacokinetics in Childhood Leukemia

Survival has been shown to improve when maintenance therapy for acute lymphocytic leukemia in children is given at night rather during the day. We examined the possibility that diurnal variation in methotrexate pharmacokinetics may contribute to this improvement. In a crossover study, we determined the pharmacokinetics of intravenous methotrexate at 10:00 and 21:00 h in six children with standard or high-risk leukemia. During the study, children refrained from concomitant drugs (6-mercaptopurine and trimethoprim sulfamethoxazole). There was a significant fall in methotrexate plasma clearance at night (from 5.6 ± 3 ml/min/kg to 4.7 ± 2.3 ml/min/kg p > 0.05). Renal clearance of methotrexate tended to decrease at night and unbound renal clearance decreased significantly (from 17.5 ± 1.7 ml/min/kg to 8.5 ± 3.6 ml/min/kg p > 0.05). Creatinine clearance did not exhibit diurnal variation, when comparing two 12-h collections, but there was a significant decrease in the nonglomerular clearance of methotrexate (from 14.8 ± 5.2 to 6 ± 4 ml/min/kg). Because it is a weak organic acid, the tubular secretion of methotrexate depends on urinary pH. At night urinary pH is more acidic. This may result in more reabsorption and hence reduced renal clearance.     

A search for a defect of proximal transport in denervated kidneys of conscious dogs

The influence of renal nerves on proximal Na+ reabsorption was studied in clearance experiments with unilaterally renal-denervated conscious dogs prepared by surgical bladder division. Two types of experiments were made : A. maximal water diuresis, and B. Total blockade of distal NaCl reabsorption with ethacrynic acid and chlorothiazide. In maximal water diuresis CH2O + CNa was used as a measure of fluid delivery to the distal nephron. At similar GFR on both sides, the proximal reabsorption estimated as GFR--(CH2O + CNa) was 38.4 +/- 5.6 ml/min for the intact and 35.9 +/- 4.2 ml/min for the denervated kidney (n = 6, difference NS). After distal tubular blockade, proximal Na+ reabsorption calculated as filtered load minus urinary excretion was 3.84 +/- 0.43 mmol/min for the intact and 3.91 +/- 0.36 mmol/min for the denervated kidney (n = 6, difference NS). The fractional reabsorption of NA+ was 64.9 +/- 1.0% for the intact and 66.9 +/- 1.1% for the denervated kidney (difference NS). In contrast to data from renal denervation studies with anaesthetized animals, the present experiments did not show any difference in proximal reabsorption between the innervated- and denervated kidney. We conclude that in absence of anaesthesia renal efferent nerves have no major effect on NaCl transport in dog proximal tubule.     

Loss of epithelial polarity: A novel hypothesis for reduced proximal tubule Na+ transport following ischemic injury

Summary Ischemia results in the marked reduction of renal proximal tubule function which is manifested by decreased Na⁺ and H₂O reabsorption. In the present studies the possibility that altered Na⁺ and H₂O reabsorption were due to ischemia-induced loss of surface membrane polarity was investigated. Following 15 min of renal ischemia and 2 hr of reperfusion, proximal tubule cellular ultrastructure was normal. However, abnormal redistribution of NaK-ATPase to the apical membrane domain was observed and large alterations in apical membrane lipid composition consistent with loss of surface membrane polarity were noted. These changes were associated with large decreases in Na⁺ (37.4vs. 23.0%,P<0.01) and H₂O (48.6vs. 36.9%,P<0.01) reabsorption at a time when cellular morphology, apical Na⁺ permeability, Na⁺-coupled cotransport, intracellular pH and single nephron filtration rates were normal. We propose that the abnormal redistribution of NaK-ATPase to the apical membrane domain is in part responsible for reduced Na⁺ and H₂O reabsorption following ischemic injury.     

Long-Term Responses to Loss of Renal Mass: Tubular Changes: A Micropuncture Study of HCO3 Reabsorption by the Hypertrophied Proximal Tubule

In rats with renal failure produced by excision of one kidney and infarction of large portions of the other kidney, given a low calcium, high phosphorus diet for 2-3 weeks, GFR was reduced by 80 percent, the fractional excretion of sodium increased from 7 to 23 percent, that of bicarbonate from 16 to 23 percent and that of water from 4 to 13 percent. Single nephron GFR in the remaining nephrons was nearly doubled and end-proximal TF/P_In was depressed from 2.3 to 1.8, and proximal TF/P_HCO3 from 0.52 to 0.35, the latter figure corresponding to an increase of absolute proximal HCO₃ reabsorption from 1.7 to 3.5 nEq/min or from 2.8 to 3.2 Eq/L of single nephron glomerular filtrate. Acute parathyroidectomy had no influence on the fall of GFR or the rise of SNGFR in the remaining nephrons and failed to cause any significant changes in proximal tubular bicarbonate reabsorption. Parathyroidectomy, on the other hand, practically prevented the rise of the fractional excretion of sodium and of water and inverted the rise of the fractional excretion of bicarbonate to a fall. The data are interpreted to indicate that secondary hyperparathyroidism in renal failure impairs distal nephron bicarbonate and sodium reabsorption and, thus, contributes to the maintenance of sodium balance, but could possibly aggravate acidosis.     

The renal type IIa Na/Pi cotransporter: structure-function relationships

The type IIa Na/P_i cotransporter mediates proximal tubular brush-border membrane secondary active phosphate (P_i) flux. It is rate limiting in tubular P_i reabsorption and, thus, a final target in many physiological and pathophysiological situations of altered renal P_i handling (1–4). In the present short review, we will briefly summarize our current knowledge about the transport mechanism (cycle) as well as particular regions of the transporter protein (“molecular domains”) that potentially determine transport characteristics.     

The glomerular filtration rate,effective renal plasma flow,and renal blood flow in the adult male chacma baboon (Papio ursinus)

Abstract: The radionuclide determination of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) has been validated in man, but not in the primate. GFR, ERPF, and renal blood flow (RBF) were measured in a group of 12 adult male chacma baboons using radiopharmaceuticals. GFR was determined using ^99mtechnetium-labelled diethylenetriamine-pentacetic acid. ERPF was measured with ¹³¹iodine-labelled hippuran. RBF, body surface area, and kidney weights were calculated using standard formulae. GFR was 49 ± 11 ml/min and ERPF was 237.9 ± 54.2 ml/min. Calculated RBF was 430.7 ± 111.9 ml/min and 507.4 ± 138.4 ml/min/100g of renal tissue. The results are in agreement with those obtained using more laborious nonradioisotopic techniques such as para-aminohippurate (PAH) and creatinine clearance and could serve as baseline normal values in the adult male chacma baboon.     

Acute Functional Adaptation to Nephron Loss: Micropuncture Studies

The renal and proximal tubule response to contralateral kidney exclusion was studied in a variety of circumstances. Recollection micropuncture studies were performed to assess the response to contralateral kidney clamping in the normal or a remnant kidney of the dog. Acute clamping of the contralateral kidney for a normal and unilateral remnant kidney resulted in marked reduction in proximal TF/P inulin ratios in the experimental kidney reflecting a 15 percent reduction in fluid reabsorption. Mean fractional excretion of sodium, potassium and water increased significantly in remnant kidney dogs but no significant change was observed in normal dogs except for potassium excretion. The marked reduction in proximal reabsorption occurred as soon as 5-15 minutes after contralateral kidney clamping and was compensated by distal reabsorption. Acute obstruction of the contralateral ureter results in a similar markedly reduced proximal tubular reabsorption. The reduction in proximal reabsorption induced by contralateral clamping occurred in the presence of reduced perfusion pressure and volume expansion and to some extent with renal denervation. When prostaglandin E₂ or acetycholine were infused prior to contralateral kidney clamping, proximal reabsorption remained at control levels and the contralateral clamping response was blocked. Similar blockade occurred after treatment with indomethacin. Acute reduction in nephron mass causes a marked depression of proximal tubular sodium and fluid absorption not obviously accounted for by hemodynamicphysical factors and humoral factors may be involved. The level of distal reabsorption to increased proximal delivery following contralateral clamping, determines the net urinary excretion.     

The neotropical shrub Lupinus elegans,from temperate forests,may not adapt to climate change

Considering that their distribution is limited to altitudinal gradients along mountains that are likely to become warmer and drier, climate change poses an increased threat to temperate forest species from tropical regions. We studied whether the understorey shrub Lupinus elegans, endemic to temperate forests of west‐central Mexico, will be able to withstand the projected temperature increase under seven climate change scenarios. Seeds were collected along an altitudinal gradient and grown in a shade‐house over 7 months before determining their temperature tolerance as electrolyte leakage. The plants from colder sites tolerated lower temperatures, i.e. the temperature at which half of the maximum electrolyte leakage occurred (LT₅₀), ranged from −6.4 ± 0.7 to −2.4 ± 0.3 °C. In contrast, no pattern was found for tolerance to high temperature (LT₅₀ average 42.8 ± 0.3 °C). The climate change scenarios considered here consistently estimated an increase in air temperature during the present century that was higher for the maximum air temperature than for the mean or minimum. In particular, the anomaly from the normal maximum air temperature at the study region ranged from 2.8 °C by 2030 to 5.8 °C by 2090. In this respect, the inability of L. elegans to adapt to increasingly higher temperatures found here, in addition to a possible inhibition of reproduction caused by warmer winters, may limit its future distribution.     

Circadian Rhythms of Renal Hemodynamics in Unanesthetized,Unrestrained Rats

Catheters were placed in the jugular vein and femoral artery of male Sprague-Dawley rats and connected to a specially designed perfusor for continuous constant infusion of 0.9% NaCl and a syringe to perform simultaneous and intermittent blood collections. This permitted continuous 24-h study of renal hemodynamics, estimated by inulin (C_in) and p-amino-hippuric acid (C_PAH) clearances; C_in represents glomerular filtration rate and C_PAH renal plasma flow. Animals were individually housed in metabolism cages in a controlled environment with light/dark 12:12 h. Urine was collected every 4 h (12:00, 16:00, 20:00, 24:00, 04:00, and 08:00) and blood sampled at the midpoint of urine collection periods. Urine and plasma sodium, potassium, inulin, and PAH were spectrophotometrically assessed. During continuous infusion of isotonic saline, C_in exhibited circadian changes with large decrease between 12:00 and 20:00 h (0.9 ± 0.2 ml/min) and acrophase at 00:30 h. Rhythmicity in C_PAH was similar with the minimum between 16:00 and 20:00 h (2.5 ± 0.3 ml/min) and peak between 00:00 and 04:00 h (acrophase at 00:25 h). Water and electrolyte excretion were also circadian rhythmic with a similar nighttime enhancement and daytime minimum. Such circadian changes persisted during continuous 0.9% NaCl infusion for several consecutive days. The unanesthetized, unrestrained rat model enables investigations in renal chronopharmacology and chronotoxicology.     

Hypoxemia increases renin secretion rate in anesthetized newborn lambs

The response of renin secretion rate (RSR) to acute systemic hypoxemia (mean arterial p0₂ 34±8 torr) was studied in mechanically ventilated, anesthetized newborn lambs 5–10 days of age (n=6). Ventilation of these lambs with room air (normoxemia) was followed by administration of low oxygen inhaled gas mixture (f_i0₂ 0.11) which was associated with no change in arterial pC0₂, pH, mean arterial pressure (MAP), renal blood flow (RBF, measured by electromagnetic flow probe), and calculated renal vascular resistance (RVR). Arterial plasma renin activity (PRA_A 4.28±1.73 to 6.46±3.00 ng AI/ml · hr), renal vein plasma renin activity (PRA_RV, 6.26±3.79 to 11.44±7.11 ng AI/ml · hr) and renin secretion rate (RSR, 19.86±21.70 to 51.32±48.54 units/min · KgBW) increased significantly (p<0.05) in response to hypoxemia. Restoration of normoxemia (arterial p0₂ 100±18 torr) was associated with significant decline in MAP (to 65±14 mmHg) and RBF (to 9.0±2.1 ml/min · KgBW) and further increases in PRA_A (to 8.98±3.40 ng AI/ml · hr), PRA_RV (to 19.04±10.62 ng AI/ml · hr) and RSR (to 88.6±77.6 units/min · KgBW). PRA_A correlated strongly with PRA_RV (r=0.84) and RSR (r=0.60) in these lambs. These results suggest that PRA_A, PRA_RV and RSR increase in response to hypoxemia in anesthetized lambs by a mechanism other than renal arterial baroreceptor stimulation, although this mechanism may be active during recovery from hypoxemia. Furthermore, PRA_A closely approximates RSR in newborn lambs under these conditions.     

Sex differences in renal angiotensin converting enzyme 2 (ACE2) activity are 17β-oestradiol-dependent and sex chromosome-independent

Background

Angotensin converting enzyme 2 (ACE2) is a newly discovered monocarboxypeptidase that counteracts the vasoconstrictor effects of angiotensin II (Ang II) by converting Ang II to Ang-(1-7) in the kidney and other tissues.

Methods

ACE2 activity from renal homogenates was investigated by using the fluorogenic peptide substrate Mca-YVADAPK(Dnp)-OH, where Mca is (7-methoxycoumarin-4-yl)-acetyl and Dnp is 2,4-dinitrophenyl.

Results

We found that ACE2 activity expressed in relative fluorescence units (RFU) in the MF1 mouse is higher in the male (M) compared to the female (F) kidney [ACE2 (RFU/min/μg protein): M 18.1 ± 1.0 versus F 11.1 ± 0.39; P < 0.0001; n = 6]. Substrate concentration curves revealed that the higher ACE2 activity in the male was due to increased ACE2 enzyme velocity (V_max) rather than increased substrate affinity (K_m). We used the four core genotypes mouse model in which gonadal sex (ovaries versus testes) is separated from the sex chromosome complement enabling comparisons among XX and XY gonadal females and XX and XY gonadal males. Renal ACE2 activity was greater in the male than the female kidney, regardless of the sex chromosome complement [ACE2 (RFU/min/μg protein): intact-XX-F, 7.59 ± 0.37; intact-XY-F, 7.43 ± 0.53; intact-XX-M, 12.1 ± 0.62; intact-XY-M, 12.7 ± 1.5; n = 4-6/group; P < 0.0001, F versus M, by two-way ANOVA]. Enzyme activity was increased in gonadectomized (GDX) female mice regardless of the sex chromosome complement whereas no effect of gonadectomy was observed in the males [ACE2 (RFU/min/μg protein): GDX-XX-F, 12.4 ± 1.2; GDX-XY-F, 11.1 ± 0.76; GDX-XX-M, 13.2 ± 0.97; GDX-XY-M, 11.6 ± 0.81; n = 6/group]. 17β-oestradiol (E₂) treatment of GDX mice resulted in ACE2 activity that was only 40% of the activity found in the GDX mice, regardless of their being male or female, and was independent of the sex chromosome complement [ACE2 (RFU/min/μg protein): GDX+E₂-XX-F, 5.56 ± 1.0; GDX+E₂-XY-F, 4.60 ± 0.52; GDX+E₂-XX-M, 5.35 ± 0.70; GDX+E₂-XY-M, 5.12 ± 0.47; n = 6/group].

Conclusions

Our findings suggest sex differences in renal ACE2 activity in intact mice are due, at least in part, to the presence of E₂ in the ovarian hormone milieu and not to the testicular milieu or to differences in sex chromosome dosage (2X versus 1X; 0Y versus 1Y). E₂ regulation of renal ACE2 has particular implications for women across their life span since this hormone changes radically during puberty, pregnancy and menopause.     

Seasonal reproductive cycle of the Galápagos tortoise (Geochelone nigra) in captivity

The reproductive physiology of nine Galápagos tortoises (Geochelone nigra) was studied from February 1988 to May 1989. The study encompassed the annual reproductive cycle to include complete mating and nesting sequences. Male (n = 4) and female (n = 5) seasonal reproductive changes were determined throughout the study with endocrine analysis and ultrasonographic examinations. Males displayed a prenuptial rise in serum testosterone (x― ±SE = 6.62 ± 0.92 ng/ml in August) during which gonadal maturation and spermatogenesis are thought to occur. The male reproductive cycle appears consistent with the prenuptial spermatogenic pattern exhibited by other tropical turtles. In the females, testosterone rose during the mating period (x― ± SE = 499.3 ± 124.6 pg/ml in October) prior to ovulation and is probably related to receptivity in the females. Progesterone was more variable, but also peaked during the mating period (x― ± SE = 1,017.2 ± 220.6 pg/ml in October) and appears related to ovulation. Estradiol rose several months prior to mating (x― ± SE = 75.5 ± 11.9 pg/ml in July) and was correlated with increased serum calcium levels. This increase in estradiol is thought to stimulate vitellogenesis several months prior to mating. Nesting occurred from November 1988 to April 1989, during which six clutches were laid. Clutch size ranged from eight to 17 eggs. Both male and female Galápagos tortoises display seasonal physiological changes that function to regulate annual reproductive patterns. Zoo Biol 17:505–517, 1998. © 1998 Wiley-Liss, Inc.     

Seasonal changes in energetics and torpor patterns in the subtropical blossom-bat Syconycteris australis (Megachiroptera)

Little is known about how animals from tropical and subtropical climates adjust their energy expenditure to cope with seasonal changes of climate and food availability. To provide such information, we studied the thermal physiology, torpor patterns and energetics of the nocturnal blossom-bat (Syconycteris australis 18 g) from a subtropical habitat in both summer and winter. In both seasons, S. australis frequently entered daily torpor at ambient temperatures between 12 and 25°C when food and water were withheld. Unlike patterns observed in temperate animals, mean minimum metabolic rates during torpor were lower in summer (0.47 ± 0.07 ml O₂ g⁻¹ h⁻¹) than in winter (0.75 ± 0.11 ml O₂ g⁻¹ h⁻¹). Body temperatures during torpor were regulated at 19.3 ± 1.0°C in summer and at 23.4 ± 2.0°C in winter. Torpor bout duration was significantly longer in summer (7.3 ± 0.6 h) than in winter (5.5 ± 0.3 h), but in both seasons, bout duration was not affected by ambient temperature. Consequently, average daily metabolic rates were also significantly lower in summer than in winter. Body temperatures and metabolic rates in normothermic bats did not change with season. Our findings on seasonal changes of torpor in this bat from the subtropics are opposite to those made for many species from cold climates which generally show deeper and longer torpor in winter and are often entirely homeothermic in summer. More pronounced torpor in subtropical S. australis in summer may be due to low or unpredictable nectar availability, short nights which limit the time available for foraging, and long days without access to food. Thus, the reversed seasonal response of this subtropical bat in comparison to temperate species may be an appropriate response to ecological constraints. Received: 6 May 1997 / Accepted: 19 October 1997     

Long-term drought effects on the thermal sensitivity of Amazon forest trees

The continued functioning of tropical forests under climate change depends on their resilience to drought and heat. However, there is little understanding of how tropical forests will respond to combinations of these stresses, and no field studies to date have explicitly evaluated whether sustained drought alters sensitivity to temperature. We measured the temperature response of net photosynthesis, foliar respiration and the maximum quantum efficiency of photosystem II (F_v/F_m) of eight hyper-dominant Amazonian tree species at the world's longest-running tropical forest drought experiment, to investigate the effect of drought on forest thermal sensitivity. Despite a 0.6°C–2°C increase in canopy air temperatures following long-term drought, no change in overall thermal sensitivity of net photosynthesis or respiration was observed. However, photosystem II tolerance to extreme-heat damage (T₅₀) was reduced from 50.0 ± 0.3°C to 48.5 ± 0.3°C under drought. Our results suggest that long-term reductions in precipitation, as projected across much of Amazonia by climate models, are unlikely to greatly alter the response of tropical forests to rising mean temperatures but may increase the risk of leaf thermal damage during heatwaves.     

Effects of extracellular nucleotides on renal tubular solute transport

A range of P2 receptor subtypes has been identified along the renal tubule, in both apical and basolateral membranes. Furthermore, it has been shown that nucleotides are released from renal tubular cells, and that ectonucleotidases are present in several nephron segments. These findings suggest an autocrine/paracrine role for nucleotides in regulating tubular function. The present review catalogues the known actions of extracellular nucleotides on tubular solute transport. In the proximal tubule, there is firm evidence that stimulation of apical P2Y₁ receptors inhibits bicarbonate reabsorption, whilst basolaterally applied ATP has the opposite effect. Clearance studies suggest that systemic diadenosine polyphosphates profoundly reduce proximal tubular fluid transport, through as yet unidentified P2 receptors. To date, only circumstantial evidence is available for an action of nucleotides on transport in the loop of Henle; and no studies have been made on native distal tubules, though observations in cell lines suggest an inhibitory effect on sodium, calcium and magnesium transport. The nephron segment most studied is the collecting duct. Apically applied nucleotides inhibit the activity of small-conductance K⁺ channels in mouse collecting duct, apparently through stimulation of P2Y₂ receptors. There is also evidence, from cell lines and native tissue, that apically (and in some cases basolaterally) applied nucleotides inhibit sodium reabsorption. In mice pharmacological profiling implicates P2Y₂ receptors; but in rats, the receptor subtype(s) responsible is/are unclear. Recent patch-clamp studies in rat collecting ducts implicate apical P2Y and P2X subtypes, with evidence for both inhibitory and stimulatory effects. Despite considerable progress, clarification of the physiological role of the tubular P2 receptor system remains some way off.