RELATION BETWEEN THE CIRCADIAN RHYTHMS OF MITOTIC RATE AND CHOLESTEROL-7α-HYDROXYLASE ACTIVITY IN THE REGENERATING LIVER

The evolution of mitoses and cholesterol-7α-hydroxylase activity were studied in parallel in the regenerating liver after a partial hepatectomy performed at two different hours of the nycthemeral period, i.e. 10 a.m. and 8 p.m.
The mitotic index was used to assess the function of cell division while cholesterol-7α-hydroxylase activity was taken as a test of the tissue-specific function. The results showed a different evolution of these two functions according to the time of the surgery. The data indicated that a mutually exclusive relationship took place between the function of cell-division and the tissue-specific function.
These observations suggested that, during liver regeneration, the derepression of the genes coding the enzymes of these two different functions takes place in a complementary fashion, at two different phases of the nycthemeral period.     

Carbon Dioxide and pH Requirements of Non-Photosynthetic Tissue Culture Cells

Maximum growth of suspension cultures of Paul's Scarlet rose required a low pH (5.2 to 5.4) during the division phase (day 0 to 7) and a higher pH (5.8 to 6.0) during the expansion phase (day 7 to 14). The fresh weight increase was reduced by approximately 22%, but the dry weight was not influenced when cells were grown for 14 days in a CO₂ deficient environment. Kinetic studies showed that the first five days of growth was the critical period of nonautotrophic CO₂ fixation when cells were grown in medium buffered at pH 5.4. The phosphoenolpyruvate carboxylase activity was highest (0.50 × 10⁶ cpm min^?1· g^?1 fresh weight) during the period when nonautotrophic CO₂ fixation appeared to be critical for growth.     

In situ and experimental evidence of the influence of turbulence on cell division processes of Peridinium cinctum forma westii (Lemm.) Lefèvre

Comparison of the diel variations of the wind intensity and the division rate (DR) of Peridinium cinctum forma westii (Lemm.) Lefèvre in situ reveals that the intensity of the wind blowing throughout the whole day does not affect the DR. On the other hand, a strong inhibitory effect is noticed when the wind episodes occur during the time period 18.00–02.00 h. Systematic hourly sampling and staining of Peridinium cells showed that nuclear division takes place between 23.00 and 02.00 h and is completed before cytokinesis begins. Thus the time period 18.00–02.00 h corresponds to the premitotic and mitotic phases of cell division, and the turbulence generated by the wind affects the process of nuclear division.

The relationship between water turbulence and the DR of Peridinium which was observed in Lake Kinneret (Israel) has been checked under experimental conditions. Peridinium was grown without shaking, with continuous rotary shaking and with intermittent shaking at 100 r min^-1. The specific growth rate (k), generation time (G) and mortality rate were followed and compared. The results obtained confirm the facts observed in situ and clarify some aspects. Intermittent shaking of 2 h day^-1 during the dark phase reveals the inhibitory effect of agitation on nuclear division. Continuous shaking causes a high rate of cell mortality. Shaking during the light period does not affect the division process.

The effect of turbulence on the DR of Peridinium explains why the timing of the bloom in Lake Kinneret is a function of the duration and intensity of the mixing period in the lake.     

Cation Metabolism in Relation to Cell Size in Synchronously Grown Tissue Culture Cell

In randomly grown tissue culture cells (mouse leukemic lymphoblast, L5178Y) the number, volume, and Na⁺ and K⁺ content increase as an exponential function with a doubling time of 11.3 hr. In synchronously grown cells the volume increase of the population and of single cells follows the same exponential function as in randomly grown cells. In contrast, the cation content fluctuates during a single cell cycle. About 1½ hr after the cell division burst (at the beginning of the S period), a net loss of K⁺ occurs for a period of about 1 hr amounting to about 20% of the total K. Over the next 5 to 6 hr, the deficit in K⁺ is eliminated. The Na⁺ content shows a double fluctuation. It falls during the cell division burst, rises when the K⁺ content decreases, falls again when K⁺ content rises, and then increases again before the next cell division burst. The net fluxes of both Na⁺ and K⁺ are very small compared to the unidirectional fluxes (less than 5%), thus small changes in the balance of influx and efflux account for the changes in cation content during the growth cycle. Both unidirectional fluxes increase dramatically (by a factor of two) about 2 hr after the cell division burst, and then remain constant until after the next cell division. The pattern of electrolyte regulation during cell division does not follow a simple function such as cell number, cell surface, or cell volume, but must be related to specific internal events in the cell.     

INTERACTIONS OF LIGHT/DARK CYCLES AND NITROGEN PULSES ON THE TIMING OF CELL DIVISION IN THE NITROGEN-LIMITED MARINE DIATOM CYLINDROTHECA FUSIFORMIS (BACILLARIOPHYCEAE)1

Cell division in most eukaryotic algae grown on alternating periods of light and dark (LD) is synchronized or phased so that cell division occurs only during a restricted portion of the LD cycle. However, the phase angle of the cell division gate, the time of division relative to the beginning of the light period, is known to be affected by growth conditions such as nutrient status and temperature. In this study, it is shown that the phase angle of cell division in a diatom, Cylindrotheca fusiformis Reimann and Lewin, is affected by the N-limited growth rate; cell division occurred later in the dark period (12:12 h LD cycle) when the growth rate was infradian (D = 0.42 d^?1) than when it was ultradian (D = 1.0 d^?1). Nitrogen-pulses did not affect the phase angle of the division gate, but could shift the time of peak cell division activity within the division gate. The effects, if any, of N-pulses were dependent upon the growth rate and the time of day that the pulses were administered. These responses indicate that the timing of cell division in this diatom is not determined solely by the zeitgeber from the LD cycle, but rather that a LD cycle control mechanism and a N-mediated control mechanism are both involved and are somewhat interdependent. In addition, an increase in protein was observed immediately after administering a N-pulse to C. fusiformis in the ultradian growth mode indicating that the accumulation of protein can be uncoupled from the cell division cycle.     

Hormonal control of deoxyribonucleic Acid and protein syntheses in pea root cortical explants

The hormonal control of DNA and protein syntheses in cortical explants taken at 10 to 11 mm from the tip of 3-day-old seedling roots (Pisum sativum cv. Little Marvel) was examined. On the auxin medium, S2M, the cortical cells began to enlarge at day 4 in culture, with no DNA synthesis or cell division throughout the 7-day culture period. With the addition of kinetin to this medium, S2M + K, the DNA content of the explants increased about three times by day 3, with further increases thereafter. This DNA increase was followed by cell division activity and subsequent tracheary element differentiation initiated at day 5. At least two divisions per parent cortical cell were required prior to this cytodifferentiation. The absolute hormonal requirements for the DNA synthesis and cell division responses were substantiated by the lack of either response in explants cultured on basal (S2M medium minus auxins) or basal + K medium for 7 days. On the auxin medium, there was no protein accumulation in the cortical explants over the 7-day period. On S2M + K medium, protein accumulation began after day 2 with a steady rate of increase until day 4, and some fluctuation thereafter. The pattern of increasing uptake of ¹⁴C-leucine was similar for days 0 to 4 in explants on either medium. After day 4 on S2M, the uptake continued to increase coincident with cell enlargement initiation, whereas on S2M + K there was a decline. Incorporation of ¹⁴C-leucine into trichloroacetic acid-precipitates of the total buffered homogenate from explants on both media exhibited a similar pattern, i.e. an increase during days 0 to 3 and then a decline to a level about three times higher than day 0. Incorporation into the homogenate soluble fraction also showed a similar pattern in explants cultured with or without kinetin. From the differences in net protein accumulation and the incorporation data, speculation on a cytokinin effect on protein synthesis and degradation rates is presented.     

Mitochondrial folates and methionyl-tRNA transformylase activity during germination and early growth of seeds

Mitochondria were isolated from the cotyledons of pea (Pisum sativum cv Homesteader) and peanut (Arachis hypogaea cv Early Spanish) seeds over a 7-day growth period. The rate of mitochondrial oxygen uptake increased 3-4-fold during the first 4 days of growth and parallel changes were observed in the respiratory control and ADP/O ratios. In both species, the total cotyledonary pool of folate derivatives increased 3-4-fold during this period of germination whereas that associated with isolated mitochondria increased 5-10-fold. Until day 3 of growth, the mitochondrial folates were principally polyglutamates of 10-formyltetrahydrofolate but between day 4 and day 7 increasing levels of 5-methyltetrahydrofolate polyglutamates were detected. Pea and peanut mitochondria contained methionyl-tRNA transformylase (EC 2.1.2.9) activity that displayed an absolute requirement for 10-formyl-tetrahydrofolate. The specific activity of this enzyme rose during germination, reaching maximal levels between days 3 and 4. Isolated pea mitochondria had the ability to incorporate [³H]leucine and [³⁵S]methionine into protein in a reaction that required ADP and malate but was strongly inhibited by chloramphenicol. Organelles isolated after 4 days of germination incorporated leucine at rates ca 5-fold greater than shown by mitochondria of 16-hour-old seedlings. The inter-relationships between respiratory activity, mitochondrial formyltetrahydrofolates and methionyl-tRNA transformylase activity suggest a role for organelle protein synthesis during germination of these legume species.     

Circadian rhythm of the mitotic activity of pulmonary interalveolar septum cells in rats of different ages]

The daily rhythm of mitotic activity in the lungs of the 20-day-dd embryo coincides with the rhythm of the adult organism. The mitotic activity of the 1-, 3- and 10-day-old animals was the maximum in the evening and the minimum-in the morning hours. A definitive rhythm of cells division (with the maximal mitotic activity in the morning and the minimal-in the evening) is established beginning from the 17th day of the postnatal development. The average mitotic activity is very high in the embryos, but it falls immediately after birth. It rises on the 3rd day, and begins to decrease again from the 7th day after birth.     

Comparison of the cytotoxic,pro-oxidant and pro-inflammatory characteristics of different oxysterols

Oxidized low-density lipoproteins play important roles in the development of atherosclerosis and contain several lipid-derived, bioactive molecules which are believed to contribute to atherogenesis. Of these, some cholesterol oxidation products, refered to as oxysterols, are suspected to favor the formation of atherosclerotic plaques involving cytotoxic, pro-oxidant and pro-inflammatory processes. Ten commonly occurring oxysterols (7α-, 7β-hydroxycholesterol, 7-ketocholesterol, 19-hydroxycholesterol, cholesterol-5α,6α-epoxide, cholesterol-5β,6β-epoxide, 22R-, 22S-, 25-, and 27-hydroxycholesterol) were studied for both their cytotoxicity and their ability to induce superoxide anion production (O₂^{⋅ −}) and IL-8 secretion in U937 human promonocytic leukemia cells. Cytotoxic effects (phosphatidylserine externalization, loss of mitochondrial potential, increased permeability to propidium iodide, and occurrence of cells with swollen, fragmented and/or condensed nuclei) were only identified with 7β-hydroxycholesterol, 7-ketocholesterol and cholesterol-5β,6β-epoxide, which also induce lysosomal destabilization associated or not associated with the formation of monodansylcadaverine-positive cytoplasmic structures. No relationship between oxysterol-induced cytotoxicity and HMG-CoA reductase activity was found. In addition, the highest O₂^{⋅ −} overproduction quantified with hydroethidine was identified with 7β-hydroxycholesterol, 7-ketocholesterol and cholesterol-5β,6β-epoxide, with cholesterol-5α, 6α-epoxide and 25-hydroxycholesterol. The highest capacity to simultaneously stimulate IL-8 secretion (quantified by ELISA and by using a multiplexed, particle-based flow cytometric assay) and enhance IL-8 mRNA levels (determined by RT-PCR) was observed with 7β-hydroxycholesterol and 25-hydroxycholesterol. None of the effects observed for the oxysterols were detected for cholesterol. Therefore, oxysterols may have cytotoxic, oxidative, and/or inflammatory effects, or none whatsoever.     

Diurnal regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity and its content in Norway spruce needles

Diurnal changes of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity and its content were measured to find the mechanism of RuBPCO activity regulation in Norway spruce needles. Both initial and total RuBPCO activities as well as the activation state had a typical pattern with two peaks in the morning and afternoon, respectively, and a midday depression. On the 19 October, RuBPCO content decreased during the day from 3.1 to 1.4 g m⁻², while on the 20 October it was approximately constant both in the morning and in the afternoon (2.7 g m⁻²). Neither initial nor total activity of RuBPCO copied irradiances. Relatively low morning and evening values of total activities indicate that nocturnal inhibitor CA1P is important in Norway spruce. However, the midday depression of total activity indicates that besides CA1P there function some other inhibitors of RuBPCO. In addition, the diminution of RuBPCO content during the day may indicate repression of its gene expression.     

Morning‐to‐Evening Differences in Oxygen Uptake Kinetics in Short‐Duration Cycling Exercise

This study analyzed diurnal variations in oxygen (O₂) uptake kinetics and efficiency during a moderate cycle ergometer exercise. Fourteen physically active diurnally active male subjects (age 23±5 yrs) not specifically trained at cycling first completed a test to determine their ventilatory threshold (T_vent) and maximal oxygen consumption (VO_2max); one week later, they completed four bouts of testing in the morning and evening in a random order, each separated by at least 24 h. For each period of the day (07:00–08:30 h and 19:00–20:30 h), subjects performed two bouts. Each bout was composed of a 5 min cycling exercise at 45 W, followed after 5 min rest by a 10 min cycling exercise at 80% of the power output associated with T_vent. Gas exchanges were analyzed breath‐by‐breath and fitted using a mono‐exponential function. During moderate exercise, the time constant and amplitude of VO₂ kinetics were significantly higher in the morning compared to the evening. The net efficiency increased from the morning to evening (17.3±4 vs. 20.5±2%; p<0.05), and the variability of cycling cadence was greater during the morning than evening (+34%; p<0.05). These findings suggest that VO₂ responses are affected by the time of day and could be related to variability in muscle activity pattern.     

Salivary Secretion under the Influence of Bright/Dim Light Exposure in the Morning and Evening in Humans

Recent studies show that bright and dim light intensities during the daytime have important regulatory functions. Our present study was performed to evaluate the effect of exposure to different light intensities during the morning and evening on salivary secretion and its sodium concentration. The study involved 6 healthy, female volunteers who were exposed to dim light (100 lx) from 7:00 to 17:00 and to bright light (3000 lx) from 17:00 to 23:00 one day, and to bright light (3000 lx) from 7:00 to 17:00 and dim light (100 lx) from 17:00 to 23:00 on the next day. We collected salivary samples every 10 minutes during 2 hours in the morning and in the evening by means of a Lashley cup. Saliva secretion was stimulated by sweet candy. The amount of saliva secreted was significantly greater in the morning under bright light exposure, while it was significantly greater in the evening under dim light exposure. We discuss these findings in terms of changes in activity of the parasympathetic nervous system (PNS) and sympathetic (SNS) nervous system produced by exposure to different light intensities at different times of the day.     

Chenodeoxycholic acid synthesis in the hamster: A metabolic pathway via 3β, 7α-dihydroxy-5-cholen-24-oic acid

The quantitative significance of the metabolism of 3β, 7α-dihydroxy-5-cholen-24-oic acid to chenodeoxycholic acid was evaluated in the hamster. A precursor-product relationship was established in this species by the finding that intravenous administration to an animal previously given cholesterol-4-¹⁴C caused a significant reduction in the specific activity of chenodeoxycholic acid. Administration of 12.9 μmole of the precursor was followed by a 10-fold increase in chenodeoxycholic acid excretion although the predominant excretory pathway was via biliary excretion as a monosulfate. The data indicate that synthesis of bile acid from cholesterol via the intermediate 3β, 7α-dihydroxy-5-cholen-24-oic acid can be a quantitatively important pathway.     

Plasma membrane lipid metabolism of petunia petals during senescence

The specific activities of 6 enzymes, which are involved in the synthesis and catabolism of membrane lipids, were monitored in plasma membranes isolated from petunia petals during senescence. These included phosphatidylinositol (PI) kinase (EC 2.7.1.67), phosphatidylinositol monophosphate (PIP) kinase (EC 2.7.1.68). diacylglycerol (DAG) kinase (EC 2.7.1.107), phospholipase A (EC 3.1.1.4) and PIP- and PIP₂-phospholipase C˙(EC 3.1.4.3). Using endogenous substrate, the [³²P]PA and [³²P]PIP₂ formation increased to 140 and 200%, respectively, of the day 1 value by 4 days after harvest. There was no significant change in [³²P]PIP formation during the same time period. On the fifth day the petals wilted and the [³²P]PA and [³²P]PIP formation declined significantly. In contrast, the [³²P]PIP₂ formation remained high in the day 5 petals. When the lipid kinase activities were assayed in the membranes in the presence of exogenous substrate the specific activity of all of the enzymes increased. and the changes in [³²P]PA production over the 5-day period were similar to those observed with endogenous substrate. When exogenous PI and PIP were added, however, there was no longer an increase in [³²P]PIP₂ formation by plasma membranes of day 4 petals and [³²P]PIP formation significantly decreased. The relative decrease in PIP and PIP₂ formation by day 4 membranes when exogenous substrate was added may have resulted from differences in the lipase activities in the day 1 and day 4 membranes. The plasma membrane A-type phospholipase activity increased throughout the 5 day period, and phospholipase C activity increased two-fold between day 1 and day 4. Such changes in the metabolism of the plasma membrane lipids during flower senescence would affect the ability of the petals to use inositol phospholipid-based signal transduction pathways.     

Morning Versus Evening Power Output and Repeated‐Sprint Ability

We investigated the effect of time‐of‐day on both maximal sprint power and repeated‐sprint ability (RSA). Nine volunteers (22±4 yrs) performed a RSA test both in the morning (07:00 to 09:00 h) and evening (17:00 to 19:00 h) on different days in a random order. The RSA cycle test consisted of five, 6 sec maximal sprints interspersed by 24 sec of passive recovery. Both blood lactate concentration and heart rate were higher in the evening than morning RSA (lactate values post exercise: 13±3 versus 11±3 mmol/L^?1, p<0.05). The peak power developed during the first sprint was higher in the evening than morning (958±112 vs. 915±133 W, p<0.05), but this difference was not apparent in subsequent sprints, leading to a higher power decrement across the 5×6 sec test in the evening (11±2 vs. 7±3%, p<0.05). Both the total work during the RSA cycle test and the power developed during bouts 2 to 5 failed to be influenced by time‐of‐day. This suggests that the beneficial effect of time‐of‐day may be limited to a single expression of muscular power and fails to advantage performance during repeated sprints.     

Na+/K+-ATPase activity in rat erythrocytes after prolonged starvation

Na⁺/K⁺-ATPase (sodium, potassium adenosine triphosphatase, EC 3.6.3.9) activity has been studied in whole erythrocytes from rats over time of total food deprivation for 1, 3, 5, 7–8, and 10–12 days with free access to water. Changes in Na⁺/K⁺-ATPase activity have been found to be phase-specific, i.e., associated with periods of certain metabolism level. After the hunger state and accommodation to endogenous nutrition (phases 0-I), from the 3rd to the 7th–8th day a period of compensated accommodation begins (phase II characterized by a stable euglycemic state, while the level of plateau of protein losses and hormonal stimulation are achieved). The Na⁺/K⁺-ATPase activity changes during the phase II were insignificant (p > 0.05), but potassium loss was observed in erythrocytes and blood plasma from the 5th day of starvation onwards. The phase III (the 10th–12th days) is an onset of the terminal period characterized by the lower activities of Na⁺/K⁺-ATPase (ouabain-sensitive activity) and Mg²⁺-ATPase (ouabain-independent activity) and by reduced sodium plasma levels that previously had remained virtually unchanged. There are considered possible causes of the observed decrease in the Na⁺/K⁺-ATPase activity during prolonged starvation, such as aging of the circulating erythrocyte population (the absence of reticulocytes and young erythrocytes), depletion of cell energy resources (hypoglycemia and glycopenia), effect of endogenous ouabain, and endotoxemia.     

The effect of indomethacin on the growth and metabolism of green alga <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> Beijerinck

The aim of the present work was to study the effect of indomethacin (IM), a pleiotropic therapeutic substance commonly used in animal systems, at concentration range of 10⁻⁸ to 10⁻³ M on the growth and metabolism of single-celled Chlorella vulgaris (Chlorophyceae). Because of the presence of the indole ring in its molecule, IM is characterized by structural similarity with natural auxins, e.g. IAA. It was found that IM influenced algal growth, macromolecular synthesis and metabolism in dose-dependent manner. IM had the highest stimulating effect on algae at 10⁻⁷ M on the 5th day of culture resulting in the increase in cell number and dry mass, DNA, RNA, proteins, phosphates, monosaccharides, photosynthetic pigments and glycolic acid content as well as protein extracellular secretion to the environment. Specific proteins from the region 20–139 kDa appeared during 10⁻⁷ M IM treatment on the 5th day of cultivation as analysed by SDS-PAGE. IM-induced photosynthetic oxygen exchange in green alga was also noted. In contrast, the treatment with IM at the highest concentration of 10⁻³ M suppressed cell division, dry mass production and decreased the level of the analysed parameters during the whole 7-day period of cultivation. Therefore, it could be speculated that IM functioned as a plant growth regulator affecting cell division and metabolism of green alga C. vulgaris.     

Studies on S-adenosylmethionine:protein-carboxyl methyltransferase in the hypothalamo-neurohypophysial complex in organ culture.

The level of protein methylase II (S-adenosylmethionine: protein-carboxyl-methyltransferase, EC.2.1.1.24) activity in the hypothalamo-neurohypophysial complex in organ culture was studied during the period of 21 days. 1. The endogenous enzyme activity, which is a measure of both enzyme and substrate protein levels, in hypothalamus is maximum at the 7th day of culture showing 100% increase, compared to the activity at 0 day. Exogenous enzyme activity in hypothalamus which is a measure of enzyme level only, did not show a peak at 7th day. Thus, this increase is a measure of enzyme level only, did not show a peak at 7th day. Thus, this increase of the activity indicates that newly synthesized neurophysin served as endogenous substrate protein. 2. Endogenous enzyme activity of cytosol fraction from anterior pituitary gland gradually increased during the culture period reaching 3-fold increase at 12th day of culture, while the exogenous activity remained unchanged. This specific increase of endogenous enzyme activity also indicates that $\text{in vivo}$ newly synthesized anterior pituitary peptide hormones serve as substrate.     

Impact of an enriched-cholesterol diet on enzymatic cholesterol metabolism during rabbit gestation

An appropriate cholesterol homeostasis is vital for the maintenance and the optimal fetal development. The cholesterol is essential for the synthesis of progesterone and 17beta-estradiol, hormones that actively participate to sustain gestation. However, the administration of 0.2% enriched cholesterol diet (ECD) during rabbit gestation significantly increased the cholesterol blood profile (total-cholesterol, LDL, HDL, esterified-cholesterol and free-cholesterol) of dams and offspring, and induced a reduction of the offspring weight of 15% as compared to the control group. Enzymes involved in cholesterol metabolism (ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase) are greatly influenced by cholesterol profile. We hypothesized that the administration of an ECD during rabbit gestation modifies the activity of those enzymes. Female rabbits (pregnant or not) were fed with a standard diet or an ECD. At term, livers (dams and offspring) and placentas were collected and ACAT, HMG-CoA-reductase and cholesterol-7alpha-hydroxylase activities were assayed. Our results demonstrate that gestation induced a reduction of ACAT activity (48.9%) in dam's liver and, an augmentation of HMG-CoA-reductase activity (142.4%) whereas it has no effect on cholesterol-7alpha-hydroxylase activity. The administration of the ECD has no additive effect on ACAT, but significantly reduced the HMG-CoA-reductase activity and cholesterol-7alpha-hydroxylase activity as compared with the pregnant control group. In placentas the ECD supplementation has an influence for HMG-CoA-reductase activity, where a 43% increased in observed. Any ACAT activity was detected in placenta and the ECD has no influence on the cholesterol-7alpha-hydroxylase activity. Whereas their offspring's liver present a reduction of ACAT and HMG-CoA-reductase activity. Gestation associated with ECD reduces significantly the HMG-CoA-reductase activity, decreasing the cholesterol synthesis, but placenta seems to compensate this effect by increasing its HMG-CoA-reductase activity.     

Circadian rhythm of cholesterol-7alpha-hydroxylase and cortisol in the African green monkey (Cercopithecus aethiops).

Cholesterol-7alpha-hydroxylase in African green monkey liver had an apparent Km of 1.65-10(-4) M cholesterol and a pH optimum of 7.4. The amplitudes of the circadian maxima of enzyme activity and serum cortisol levels were significantly greater in vervets than in grivets. Fluctuations in enzyme activity and cortisol levels during the circadian cycle were positively correlated (r = 0.89). Enzyme activities and hormone levels were 2.7-fold lower over a 24-h period in the grivet than in the vervet. Cholesterol feeding reduced the enzyme activity by 40% and serum cortisol was reduced to 38% of control levels at the diurnal peak. Serum glucocorticoids may be important physiological regulators of cholesterol-7alpha-hydroxylase in non-human primates. The concentration of cortisol and its time of release appear to be factors in the hyperresponsive trait of grivets. Genetic differences between vervet and grivet races may account for differences in the amplitude and timing of the circadian rhythm of cholesterol-7alpha-hydroxylase activity possibly influenced by cortisol.