superficial proximal tubular length, nephron number and kidney weight

J. Physiol. (1977), 272, pp. 573589 573 With 8 teztft urea Printed in Great Britain DEVELOPMENTAL CHANGES IN NEPHRON NUMBER, PROXIMAL TUBULAR LENGTH AND SUPERFICIAL NEPHRON GLOMERULAR FILTRATION RATE OF RATS BY SIDNEY SOLOMON From the Department of Physiology, University of New Mexico, School of Medicine, Albuquerque, New Mexico 87131, U.S.A. (Received 4 January 1977) SUMMARY 1. Postnatal development of single nephron glomerular filtration rate, superficial proximal tubular length, nephron number and kidney weight have been studied in Sprague Dawley and in Wistar rats. 2. Superficial tubular length is a nonlinear function of body weight or age. There seems to be a rapid growth until animals weigh about 15 g in Wistar rats. In this strain, growth is slower thereafter. This difference is not as evident in Sprague Dawley rats. 3. Nephron numbers increase over the same period at which rapid tubular growth occurs. 4. Sprague Dawley rats have somewhat fewer, but longer, proximal tubules than do Wistar rats. 5. In all animals weighing more than 1 g, SNGFR is linearly related to ikeight. For younger, smaller Sprague Dawley rats, the same linearity holds over the age range studied older than 2 days of age. In Wistar rats, SNGFR relative to weight is less in young animals. 6. By relating SNGFR to total kidney GFR, evidence is obtained that maturation of renal function also involves a greater increase in filtration by superficial than by juxtamedullary nephrons. INTRODUCTION There are several reasons for carrying out the experiments to be described. Although the postnatal pattern of development of glomerular filtration (GFR) has been studied in quite a few mammalian species (Horster & Lewy, 197; Horster & Valtin, 1971; Potter, Jarrah, Sakai, Harrah & Holliday, 1969; Rubin, Burch & Rappaport, 1949; Solomon & Capek, 1972a, b; Spitzer & Brandis, 1974, for example), relatively few micropuncture investigations have been carried out. In the rat, a relatively restricted study on changes in superficial single nephron glomerular

574 S. SOLOMON filtration rate (SNGFR) has been done in this laboratory (Solomon & Capek, 1972a). Spitzer & Brandis have done a more complete micropuncture investigation of changes in SNGFR in baby guineapigs (Spitzer & Brandis, 1974). Sin,e the rat is the experimental animal used in so much micropuncture work, it was thought desirable to make a more complete study of developmental changes in SNGFR over an extended time period, not only during the immediate postnatal period, but also during the time of mature growth. A second issue to be considered in these studies is concerned with the fact that there are differences in the absolute values of SNGFR reported from different laboratories. One possibility which has been considered and shown in one study (Brenner & Daugherty, 1972) is that differences exist in different strains of rats. It was decided, therefore, to examine if the developmental patterns in Sprague Dawley and Wistar rats might be different. In addition, some attention has been paid to developmental changes in morphological parameters such as glomerular number (Bonvalet, Champion, Wanstok & Berjal, 1972) and tubular lengths and diameters (Solomon, 1974; Spitzer & Brandis, 1974; Wahl & Schnermann, 1969), but few studies exist which relate these changes to development of filtration. For example, de Rouffignac & Bonvalet found a relationship between proximal tubule length and SNGFR (de Rouffignac & Bonvalet, 197). Thus, they observed that juxtamedullary nephrons were longer and had a higher SNGFR than did superficial nephrons of the same kidney. Although the relationship between proximal tubule length and SNFGR has been studied in the guinea pig (Spitzer & Brandis, 1974), it is not known in the rat what the relationship between proximal tubule length and SNFGR is during the developmental process. It was decided to do a more complete study of changes in glomerular filtration, not only during certain aspects of immature development, but also extending into mature growth. This study reports on (a) the magnitude of superficial SNGFR in animals older than 2 days, (b) on the relationship between SNGFR and GFR, (c) on changes in the number of glomeruli during maturation, and (d) on changes in proximal tubular lengths over the same ranges of development and growth. In addition, the developmental patterns have been examined in both Sprague Dawley and Wistar rats. METHODS All of the experiments to be described were carried out on Wistar and Sprague Dawley rats obtained from Simonsen (Gilroy, California). In most studies, pregnant mothers were shipped to this laboratory. The pups were weaned 21 days after birth and were studied at varying times thereafter. Animals were not used unless there

RENAL DEVELOPMENT OF IMMATURE RATS 575 were at least ten and no more than thirteen siblings in a litter. Some data on Sprague Dawley rats were taken from studies carried out for other purposes. These studies were done on mature animals of known age which were obtained from Simonsen and then were acclimated to this laboratory for at least 1 week before use. No physiological studies are reported from these animals. They have been used only to provide additional data on superficial nephron proximal lengths. Because the rate of growth of these animals may have been different from that in our own laboratory, the length data are plotted as a function of body weight rather than age. Micropuncture studies and tubular measurements The preparative procedures used for measuring SNGFR and whole kidney glomerular filtration GFR have been described before in publications from this laboratory (Solomon, 1974; Solomon & Capek, 1972a; Sonnenberg & Solomon, 1969). After animals were anaesthetized, a jugular vein was cannulated for infusion of Ringer containing [3H]inulin; the ureter of the left kidney was catheterized for urine collection into preweighed plastic tubes. The left kidney was then mounted in a plastic cup, bathed with oil and illuminated for micropuncture studies. Collections were made under 'controlled suction'. Although controversy exists about the validity of measuring SNGFR by sample collection from the proximal tubule (Davis, Schnermann & Horster, 1972; Knox, Ott, Cuche, Gasser & Haas, 1974; Navar, Burke, Robinson & Clapp, 1974; Schnermann, Davis Wunderlich, Levine & Horster, 1971), it was necessary to use this site for sampling. In the youngest Wistar rats used in these studies (2 days old), there are but a few distal tubules on the surface. Additional problems arise because distal tubules are very narrow, thereby making puncture very difficult, and also because tubular fluid flow rates are very slow. Therefore, the use of distal tubules was precluded for measuring SNGFR. Micropuncture and clearance studies were started two hours after start of Ringer infusion. Five to nine samples were taken from each animal. SNGFR was measured by taking timed collections of fluid and transferring them in toto to vials containing a scintillation cocktail. Radioactivity was later counted using a Packard TriCarb liquid scintillation counter. Total GFR was measured as described before. At the termination of the studies, tubules were filled from a latexfilled pipette. The kidney was removed, digested in hydrochloric acid, washed with tap water and stored under tap water for one to three days. The casts of the tubules were dissected free and the length of the pars convolute only was measured using a microscope fitted with ocular micrometer. Measurements were accepted only when the tubules were attached to the glomeruli from which they originated. This segment was used since it was found possible to identify the separation between the straight and convoluted portions of the proximal tubule, but it was often difficult to see a clear separation between the end of the pars recta and the start of the thin loop of Henle. Some of the data obtained on tubule lengths of Sprague Dawley rats are taken from information obtained in a previous investigation (Sonnenberg & Solomon, 1969). Glomerular counts, kidney weigh and body weigts Other rats were used for the determination of glomerular numbers. Counting of glomeruli was started when animals were fivedays old and were continued at varying time intervals thereafter. Siblings from single litters were counted at varying times after birth in order to reduce any potential bias which could result from litter differences. Animals were removed from their cages in the late forenoon and were weighed and anaesthetized. The kidneys were removed, decapulated, weighed and macerated in 5 % hydrochloric acid at 37 'C, following in general the modification by Bonvalet,

576 S. SOLOMON Champion, Wanstok & Berjal (1972) of the method of Damadian, Shwayri & Bricker (1965). It was necessary to modify the procedure further, however, in this laboratory. When kidney weight (KW) was less than 5 g, effective glomerular separation could be achieved after 15 min of digestion. With kidneys weighing more than this, the central portion of the kidney could not be macerated easily with this time of digestion. Increasing the time of incubation resulted in having the outer cortex overdigested. It was decided to cut the kidneys into more than one piece when this critical weight was exceeded and to add together parts of broken glomeruli during the counting procedure. Glomerular parts were easily identified since they were more yellowbrown than tubule fragments, probably because of containing acid haemoglobin. An estimate was made of the fraction of a total glomerulus which was contained in each fragment. The fragments then were added together to the nearest glomerulus. The maximum number of glomeruli represented by fragments was always less than 2 %. This process did not result in glomerular counts different from those obtained by counting kidney sections (unpublished data of Roy Horst, Department of Anatomy, University of Vermont), and counts will be shown to be consistent with results obtained by others. After digestion, the kidney or kidney pieces were washed and transferred into 1 ml. volumetric flasks halffilled with tap water. They were kept in the refrigerator for 24 hr and then separated by gentle shaking. They were brought to volume and at least two 5 ml.. aliquots were then counted. Not all glomeruli were of the same size. One could not separate out immature and mature glomeruli by these techniques, although such a separation would have been desirable. Duplicate counts were accepted as valid when they did not differ by more than 1%. Most duplicates were actually within 5 % of each other. If, however, the differences between successive counts were excessive, counting was repeated until consistency was obtained. There are precautions which must be taken in order to get reasonable results with this method. Problems with digestion have been considered above. If, in the course of maceration, shaking is excessive, both tubules and glomeruli become highly fragmented and the apparent glomerular count is decreased. Further fragmentation can be caused by using pipettes with narrow openings, and measuring pipettes should be used with opening bores greater than 1 mm. Samples should be removed within 15 sec after termination of mixing. Differences in counts have been found between samples taken from the bottom and the top of the flask at longer times probably because of gravitational sedimentation of the glomeruli. High counts of aliquots taken from the bottom of the flask are particularly evident in samples obtained from kidneys of older, larger animals. As a final precaution, one should transfer the aliquot as rapidly as possible or else sedimentation within the pipettes can produce errors. RESULTS Anatomical considerations Fig. 1 shows the relationship between left kidney weight and body weight (BW) for both strains of rats. For both strains, no significant differences between right and left kidneys have been found, and no difference was found in the relationship between kidney weight and body weight in females. The best mathematical fit of the relationship between kidney weight and body weight has been shown to be that of a double reciprocal (Solomon & Bengele, 1974). For female Sprague Dawley rats, the equation is found to be 1/KW = 449+151/BW and for males, it is

RENAL DEVELOPMENT OF IMMATURE RATS 577 1/KW = X321 + 156/BW. For Wistar females, it is 1/KW = 466+153/ BW and for males, it is 1/KW = 483+141/BW. Using the standard errors of the regression coefficient (3.65 for male Sprague Dawley and 48 for male Wistars) it can be calculated that the kidneys of male Sprague Dawley rats are significantly, if only slightly, larger than those of male Wistars at body weights of 25 g or more (P <.5). No significant difference could be established for female rats. 1 8 1 6,11 4 g 1 _ C 8 2 6 4 2, r it o. 1 2 3 Body wt. (g) @ 4 5 I 12. 1 8 * *.Ad*/ Ow..j#. > 6 c U4 21 4 o 1 1~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1 2 3 Body wt. (g) Fig. 1. Relationship between kidney weight and body weight. Top panel shows the relationship between kidney weight and body weight for male rats. The lower panel shows the same relationship for female rats. Note change of scales between upper and lower panels. Filled circles show data obtained from Wistar strain rats, open circles show data from Sprague Dawley strain.

578 S. SOLOMON Since in Wistar rats, it has been shown that kidney weight to body weight ratio is inversely relate& to growth rate for up to at least 4 days postnatally (Solomon & Bengele, 1973; Solomon & Capek, 1972b), the possibility that the male Sprague Dawley rats grow slower than the Wistars was examined. No such difference in growth rate was detected (Fig. 2). 5 K 41 3F cm 3: co 2F 8 * T a 11 U 4. ). so i o 2 4 6 Age (days) 8 1 Fig. 2. Relationship between body weight and age of neonatal rats of the Wistar strain (filled circles) and of Sprague Dawley strain (open circles). Two obvious possibilities exist as to differences in the developmental processes which underlie the difference in kidney weight. There could be different numbers of nephrons in the two strains, or else the mass of each

RENAL DEVELOPMENT OF IMMATURE RATS 579 nephron could be different. In Fig. 3, the number of glomeruli as a function of age is shown for both species. No sex difference exists in the number of glomeruli for either strain of rat. In both strains of rats, the developmental patterns are similar. Thus, there is an increase in the number of glomeruli 35 r 3k 25p. * O To * * O a Go. 8 ) 2 F Cu ' 15 F C,, 1. E C X 35 o3 I.. I 25 o I o. I~ no 8 2k o 15k o ic 1 2 3 4 5 6 Age (days) 7 8 9 1 Fig. 3. Dependence of glomerular number of age on rats. Upperpanel represents data from Wistar strain and lower panel from Sprague Dawley. Data from males are filled circles and open circles are from females. until animals are 346 days old. If one puts the data in terms of kidney weight, the full complement of glomeruli is attained when the kidney has reached a weight between 7 and 8 g for Wistar rats. Referring the number of glomeruli to kidney weight may be more appropriate since, in unpublished studies, we have found that fast growing Wistar rats reach 24 PHY 272 I

58 S. SOLOMON their full complement at the same kidney weight but at an earlier age than normally growing rats. Furthermore, one cannot ascribe the difference in kidney weight between male rats of the two strains to differences in the number of nephrons. On the contrary, a small but significant difference in nephron number in the opposite direction to that which one might expect on the basis of kidney 7 6 E 8 o. 4 _ *O o o S o. CD 2 I~~~C 5 1 15 2 25 3 35 4 Body wt. (g) Fig. 4. Changes in proximal tubular length with body weight of superficial nephrons of left kidney of Wistar (filled circles) and Sprague Dawley (openl circles) strains of rats. weight, is found in these studies. Sprague Dawley rats have a lesser number of glomeruli than do Wistars despite the fact that they have bigger kidneys. If one fits the developmental curves with couple reciprocal curves, the number of nephrons of rats over 15 g in weight is 31,5 + 535 for Wistar, and 28,8 + 14 for Sprague Dawley. Although this is but a difference of 27 nephrons, it is a highly significant difference (P <.1). The right kidney has more nephrons than the left by a small number. Thus, in Sprague Dawley rats, glomerular counts of the right kidney are greater by 132±+49 (P <.2). In Wistar rats, the difference is 123+ 4 (P <.1). One would expect that the differences between the two strains of rats could be explained by differences in nephron mass. Fig. 4 shows the developmental pattern of superficial proximal tubular length. Because some of the

RENAL DEVELOPMENT OF IMMATURE RATS 581 data is taken from animals used for other studies (Sonnenberg & Solomon, 1969), ages and kidney weights are not always available and, as a result, the lengths are expressed as a function of body weight. By referring, however, to Figs. 1 and 2, it is possible to approximate the corresponding ages from the body weights. As has been shown previously (Solomon, 1974), there are two apparent phases of growth of the tubules in Wistar rats, a rapid phase until animals are between 125 to 15 g weight, followed by a 4 3 U 2 z 1 _ * o. I, I,, I 5 1 15 2 25 3 35 4 Body wt. (g) Fig. 5. Relationship between SNGFR and body weight of Wistar (filled circles) and Sprague Dawley (open circles) strains of rats. phase of slower growth. This weight would correspond to an age of 35 4 days. Statistical analysis of the regression between body weight and tubular length shows that in Sprague Dawley rats, tubules are longer than in Wistar rats for animals over 15 g body weight. Sprague Dawley rats have the relationship, length = 3938 + 3367 ( ± 3.7) body weight. The number in parenthesis indicates the standard error of the regression. For Wistar rats, the relationship is tubular length = 1642 + 8 894 (± 2.649) body weight. Over the range of mature animal weights studied, tubules from Sprague Dawley rats are significantly longer than those from Wistars. The absence of a significant slope between body weight and tubular length would show that in this strain there is also a change in growth rate in mature animals as compared to growth rates in young pups. 242

582 S. SOLOMON Development of glomerular filtration rate When total kidney GFR of animals weighing over 15 g is expressed in terms of body weight, no significant differences are evident. In Sprague Dawley rats, filtration rate is 3 39 ml./min. kg ( ±.143), while in Wistars, it is 377 (+ 251) ml./min. kg. Nor are there any differences when expressed in terms of kidney weight (832 + 59 ml./min. g for Sprague o1 r n 1 4 C 9 F 8 1 CU E,. CU. Q 7 F 6 4 1 11 51 31 2, a co I 1 2 3 4 5 6 7 Age (days) Fig. 6. Apparent number of nephrons (obtained by dividing GFR by SNGFR) as a function of age. Filled circles show data from Wistar strain. Open circles show data from Sprague Dawley strain. Circled points indicate data from Wistar strain rats weighing less than 5 g. o * I I 7 Dawley versus 893 + 51 ml./min. g for Wistar rats). Measurements in GFR of animals younger than this show the previously described age dependent increase (Horster & Lewy, 197; Horster & Valtin, 1971; Solomon & Capek, 1972b).

RENAL DEVELOPMENT OF IMMATURE RATS 583 As far as SNGFR is concerned, there is an increase as the animals develop. As shown in Fig. 5, no obvious difference exists between the two different strains for older animals. In order to compare these data in terms of body weight to those of other workers they will be discussed further. The calculation of SNGFR per 1 g body weight for animals heavier than 15 g is 1'3 + 8 nl./min for Sprague Dawley rats and 119 +.5 nl./min for Wistar rats. For Sprague Dawley rats weighing more than 5 g, there is no significant difference in the relationship. In Wistar rats, SNGFR/I g body weight is less in neonates than in mature animals. In animals weighing 575 g, it is 67 nl./min; for animals weighing 75 1 g, it is 1*9 nl./min. Although the last figure is not statistically different from SNGFR of animals weighing over 15 g, at lower weights the difference is probably valid (P <.5). Since it has been shown that there is a redistribution of filtration to superficial nephrons during early development (Solomon & Capek, 1972b; Spitzer & Brandis, 1974), it was decided to determine what the developmental pattern of this redistribution is in rats. To get at this problem, an apparent number of nephrons was calculated by dividing total kidney GFR by SNGFR. Fig. 6 shows the apparent number of nephrons as a function of age. No difference between strains is seen. A marked decrease occurs in the apparent number of nephrons from the time animals reach an age of about 4 days. Older rats do not seem to show any decrease in apparent number of nephrons. Fig. 6 also includes nephron numbers on a few Wistar pups weighing less than 5 g and aged 222 days old. Despite the large scatter of the data, the apparent nephron number is less in some animals when they weigh less than 455 g than in the age range of 254 days. DISCUSSION Following birth, the GFR per gram kidney weight shows an increase until some postnatal time at which the value is constant. Changes in GFR have generally been attributed to two factors: (a) an increase in the number of filtering nephrons, and (b) an increase in the amount of filtrate per nephlon. The results obtained in this study generally support both of these views. Thus, we find an increase in the number of nephrons over about the first 5 days postnatally and secondly, we find an increase in absolute values of SNGFR throughout the age span and range of animal weights used in this study. These events occur in both strains. The results of this study also support a second conclusion which can be made regarding development of overall renal function. The fact that the apparent number of nephrons decrease in animals weighing over 5 g until they reach about 4 days of age indicates that superficial nephron GFR increased relatively

584 S. SOLOMON more than did juxtamedullary nephron glomerular filtration rate (JNGFR) over this period of development. (The increase in apparent number of nephrons in the smaller animals probably reflects the developmental increase in the number of filtering nephrons.) After that, filtration rates of both populations of nephrons increase proportionally. Total GFR/g kidney wt. (or GFR expressed as a function of body weight) reaches stable values before the changes in distribution of nephron filtration rate are completed (Horster & Valtin, 1971; Sonnenberg & Solomon, 1969). As a result, it should be recognized that just measuring total GFR per se need not indicate maturation. These studies show that development of changes in distribution of GFR are also a part of the maturation process. CD 22 2 18 _ 16 C. C 14 12 E 1 C c8 *. 6 <4 2_ 1~~~~ 5 1 15 2 25 3 35 Body wt. (g) Fig. 7. Apparent number of nephrons (calculated by dividing GFR by SNGFR) for guineapigs of different weights. Points calculated from data of Spitzer & Brandis (1974). A similar conclusion is implicit in the work of Spitzer & Brandis on the guineapig (Spitzer & Brandis, 1974). They calculated that JNGFR (juxtamedullary nephrons) increased for the first 15 days postnatally, and then remained constant while SNGFR increased continuously over the 3 postnatal days of their study. If one calculates an apparent number of nephrons as a function of animal weight, there is an increase over the time range of that study (Fig. 7) (in contrast with the findings in rats, see Fig.6). In a comparable micropuncture investigation by Horster & Valtin on the dog, an apparent number of nephrons calculated from their data do not

RENAL DEVELOPMENT OF IMMATURE RATS 585 show any age dependent changes (Horster & Valtin, 1971). The values are so scattered, however, that only a marked trend would be evident. Studies on different species therefore show that there are at least quantitative differences in the pattern of development. If, in fact, the ratio of filtration by superficial and juxtamedullary nephrons remains constant during development of the dog, there may even be qualitative differences between species. Both strains of rats do not develop in quantitatively identical patterns. In the youngest and smallest Wistar rats which were punctured, the SNGFR when normalized in terms of body weight was less than that of the adult. (As will be discussed later, an adult is defined by me as an animal being more than 4 days old or weighing more than 15 g.) On the TABLE 1. SNGFR of Wistar and Sprague Dawley rats as found in various laboratories Wistars Weight range SNGFR (g) (nl./min. 1 g) Reference 254.4* 93 ± 3t Coelho, J. B. (1973) t 122+ 9 Horster, M. & Lewy, J. E. (197) 1931 14.2+5 Solomon, S. (1974) 151365 137+ 9 Damadian, R. V., Shwayri, E. & Bricker, N. S. (1965) 14±6 Coelho, J. B., Chien, K.C.H. & Bradley, S. E. (1972) Sprague Dawleys Weight range SNGFR (g) (nl./min. 1 g) Reference 246 1 9 + 9 Bartoli, E. & Earley, L. E. (1973) 318.1* 112+5 Coelho, J. B. (1973) * Only average body weights given. t Values obtained on MunichWistar strain. Weights of animals not given in the data. other hand, SNGFR per 1 g body weight of Sprague Dawley rats was the same over all ranges of weight and age used in these studies. These studies indicate that during development of Wistar rats, the filtration rate of individual nephrons first show a disproportionately large increase after the onset of filtration and later show slower changes which may reflect general aspects of growth; i.e. the continuous increase in body size and organ weights which accompany aging. Whether comparable changes also occur in Sprague Dawley rats cannot be concluded from this study since

586 S. SOLOMON such a developmental pattern could have taken place at a postnatal time earlier than that of the animals used in this study. Another point of interest is that the mature stable SNGFR, when expressed in terms of body weight, is greater in Sprague Dawley rats than in Wistars. Brenner obtained comparable results using the MunichWistar strain (Brenner &. Daugherty, 1972). Table 1 shows a summary of data (as originally presented or recalculated) which have appeared in the literature and wherein animal weights were published so that it has been possible to normalize SNGFR in terms of body weight. It is clear that Wistar rats, except for the Munich strain, do have higher SNGFR than do Sprague Dawley in other laboratories. Also, this way of normalizing the data results in a marked reduction in quantitative differences between laboratories (Wright & Giebisch, 1972). In contrast to the above, total kidney GFR is not significantly different between strains although Wistar rats do have a higher mean GFR. If one considers that GFR is about the same per kidney in both strains, while nephron number and SNGFR are lower in mature Sprague Dawley rats, one would predict that Sprague Dawley rats would have a greater JNGFR. Another possibility is that the number of juxtamedullary nephrons is greater in Sprague Dawley rats and that they have a higher nephron GFR. This point has not been established. The fact that nephron numbers are lower in Sprague Dawley rats while the kidney weight is larger in older male rats and the same as Wistars in females, can be explained by the observation that the superficial proximal convolutions are longer. If the remainder of the nephron has comparable proportions in both strains, the Sprague Dawley nephrons would be longer overall. It is of interest that the early increases in length do not seem to be much different in the two strains of rats, but that it is during mature stages of growth that the tubule lengths become more disparate. Accordingly, it is only when male animals are over 25 g in body weight that the kidney weight of male Sprague Dawleys is greater than that of Wistars. When SNGFR is related to proximal convoluted tubular length, one does not find a constant relationship in the rat. Whereas SNGFR is a linear function of body weight in older animals (Fig. 5), proximal tubular length is a nonlinear function of body weight as shown in Fig. 4. Statistical analysis of these data in this Figure shows that they are better fitted by a hyperbolic function. (An Ftest using a comparison of mean square deviations from each regression was used for computing this difference between fits.) As shown in Fig. 8 for those animals where enough data were obtained to calculate SNGFR per unit length of tubule, it would appear that a constant relationship is found at the early stages of development. Mature growth is characterized by a disproportionate increase in SNGFR. This

RENAL DEVELOPMENT OF IMMATURE RATS 587 finding is in contrast with the developmental pattern of guineapig (Spitzer & Brandis, 1974). Since this species was studied only for the first 3 days postnatally, it is possible that dissociation of SNGFIR and tubular length may have occurred at a later stage of growth. The observation that SNGFR is proportional to tubular length in a single kidney (de Rouffignac & Bonvalet, 197) does not always apply in comparisons between animals of the same species at different ages. 1 _ 8 E~~~~~~~~~~~~~~~~~.~ E 6 CL O L_ CD 4 Z D.~~~ 2 ~~~~~~~ 5 1 15 2 25 3 35 Body wt. (g) Fig. 8. Relationship between SNGFR per mm proximal tubular length and body weight of rats. Filled circles show data from Wistar strain and open circles data from Sprague Dawley strain. As a final point in this discussion. I should like to consider the definition of maturity of kidney function in the rat. As can be seen in this study, there are transitions in developmental patterns with respect to distribution of SNGFR, growth rate of tubules, number of nephrons. Although some kidney functions show mature characteristics at relatively early ages (see Bengele & Solomon, 1974, for some references on this matter), others show changes at a postnatal age of about 354 days or when animals are about 12515 g in weight. In our laboratory, we have found that other functions which show maturity at this time are the response to blood volume expansion (Bengele & Solomon, 1974) and basal fractional excretion of sodium (Solomon, Bengele & Smith, 1974). It would therefore appear that renal maturity does not occur until animals are at least 4 days of age or 15 g in weight.

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