After orchiectomy (OX), domestic cats gain, on average, 25–30 % in body weight, the majority of which is from accretion of body fat. The responsible physiological perturbation is not known with certainty. Kanchuk et al. (Reference Kanchuk, Backus and Calvert1) suggested that food intakes of male and female cats are modulated by oestrogens, so that after gonadectomy, inhibition of food intake by oestrogens is diminished, as has been reported in female rodents. Indeed, the most potent of endogenous oestrogens, oestradiol (E2), circulates in the plasma of sexually intact male cats in concentrations similar to those in females except during the ovarian follicular phase(Reference Perez, Conley and Dieter2). Oestrogen in males may be produced by the testis and extragonadally from activities of aromatases on testosterone and of other enzymes on oestrone (E1) and androstenedione(Reference Vermeulen, Kaufman and Goemaere3). In later investigations, Cave et al. (Reference Cave, Backus and Marks4, Reference Cave, Backus and Marks5) evinced an effect of oestrogen on food intake in cats. These investigators found that daily administration of E2 prevented post-neutering weight gain in male and female cats. They further found that E2 administration also reduced food intake in overweight gonadectomised males and females. It appeared to them that the overweight condition potentiated the food intake inhibition caused by the oestrogen.
Cave et al. (Reference Cave, Backus and Marks4, Reference Cave, Backus and Marks5) reported that exogenous E2 effectively inhibited food intake at a very low dose. This dose did not appear to evoke oestrus and only modestly changed vaginal cytology from the anoestrus condition in ovariectomised cats. These findings prompted speculation that oestrogen administration might be a useful adjunct for weight-loss treatment. With respect to this prospect, the physiological relevance of the previously described minimally effective dose of E2 for inhibiting food intake was presently investigated. Towards this aim, it was determined whether OX reduced plasma E2 concentrations in males and whether the E2 dose restored pre-OX plasma E2 concentrations in cats. Additionally, because adipose is a reputed source of oestrogen(Reference Wake, Strand and Rask6), it was determined whether plasma E2 and its potential circulating precursor, E1, are increased during post-OX gain in body fat mass.
Experimental methods
Animals
A total of six purpose-bred adult (1·3–1·6 years), male, domestic, short-hair cats (4·3–7·1 kg) were studied. The cats were housed in a controlled temperature (23–27°C) and light (12 h light–12 h dark) facility individually in lodges (1·1 × 1·6 × 2·4 m) except for 30 min daily for socialisation in a common room. Fresh water and a commercial extruded, dry-type diet, formulated for maintenance (17·9 kJ/g, Special 33; Royal Canin USA, St Charles, MO, USA) were presented daily for ad libitum consumption. Body weight and food intake were determined weekly and daily, respectively. The study was approved by the Institutional Animal Use and Care Committee of the University.
Experimental protocol
Following a pre-OX adaptation period of 3 weeks, food was withheld overnight, body fat mass was estimated and cats were neutered by the standard open technique after pre-medication (acetylpromazine (0·02 mg/kg); atropine sulphate (0·04 mg/kg); buprenorphine (0·01 mg/kg) and anaesthesia (ketamine (10 mg/kg); diazepam (0·5 mg/kg)). Body fat mass was estimated as described previously from a dilution of subcutaneously administered sterile salinated (9 gNaCl/l) 2H2O (0·4 g/kg) in body water(Reference Backus, Cave and Ganjam7).
Daily diet presentation to each cat after OX was limited to 110 % of its pre-OX mean daily intake. The restriction was to prevent substantial gain in adipose and thereby theoretically limit adipose metabolism of oestrogens. For 3 weeks, beginning the day after OX, interscapular subcutaneous injections of sterile-filtered 17β-E2 (0·5 μg) dispersed in sesame oil vehicle (0·1 ml) were given daily to three cats, while injections of vehicle alone were given to the other cats. After 3 weeks, the injection treatments were crossed over and continued an additional 3 weeks (Fig. 1). Subsequently, the cats were randomly reassigned to two groups of three cats. Diet was continuously presented to one group, while to the other group, diet continued to be limited to 110 % of pre-OX intake. After 8 weeks, body fat mass was estimated in all cats. Food presentation was then restricted until body weights of the cats were reduced to near pre-OX weights at rates not greater than 2 % body weight/week. The diet presentation scheme was then crossed over, and 8 weeks later, body fat mass was again estimated.
Order of experimental trials, procedures and sampling. OX, orchiectomy; vh, vehicle; E2, oestradiol. *Each cat was daily presented with 110 % of their pre-OX mean daily food intake.
During each of the last 3 weeks of each experimental period, blood (5 ml) was collected by jugular venepuncture and added to tubes containing anticoagulant (4·5 mg EDTA [K3]; Kendall Monoject, Tyco HealthCare, Mansfield, MA, USA). Plasma was extracted after centrifugation (1200 g, 10 min) and stored at − 20°C until assayed for oestrogen content. During E2 and vehicle administrations, blood was sampled 4 h after injections. In a separate trial, the appropriateness of this sampling time was evaluated by determining E2 in the plasma of blood collected from each cat before and at 2, 4, 8, 16 and 24 h after a single E2 injection.
Plasma oestrogens
Plasma E1 and E2 concentrations were determined by a method developed for use on human plasma(Reference Yasui, Yamada and Kinoshita8) with the following modifications: plasma samples (1 ml) were solid-phase extracted, dried at 35°C by centrifugal evaporation, reconstituted in 2-propanol and injected (40 μl) on an HPLC column (Capcell Pak NH2 UG80 S5; Shiseido, Tokyo, Japan). E1 and E2 fractions were isocratically eluted at 1 ml/min with a mobile phase, 2-propanol–hexane (3:7, v/v, HPLC grade; Fisher Scientific, Fair Lawn, NJ, USA), for which the residue left after drying did not interfere with RIA of the oestrogens. After drying by centrifugal evaporation, the oestrogens in the fractions were quantified by RIA using half the volume of reagents of commercial kits (E1 RIA, DSL-8700; Diagnostic Systems Laboratories, Webster, TX, USA and ImmunoChem™ Double Antibody, 17β-Oestradiol; ICN Biomedicals, Cosa Mesa, CA, USA, respectively). For the E1 RIA, kit standards were substituted with dried aliquots of E1 diluted in ethanol. The dried E1 standards and plasma fractions were reconstituted in 25 μl buffer, in which γ-globulin was substituted with gelatin from porcine skin (type A, Sigma; St Louis, MO, USA). For the E2 RIA, kit standards were used and plasma fractions were reconstituted with 25 μl of standard that did not contain E2. Tritiated E1 and E2 were not added to plasma samples as internal standards. The specific activities of commercially available labels were too low for use in plasma samples with low E2 concentrations. However, recoveries of radioactivity (400–800 Bq) of freshly fractionated tritiated labels (GE Healthcare, Amersham, Buckinghamshire, UK), [2,4,6,7-3H]E1 (14 GBq/mg) and [2,4,6,7,16,17-3H]E2 (20 GBq/mg), which were added to the samples of feline plasma (n 3), were high, 78 (sd 4) and 72 (sd 3) %, respectively.
Statistics
Means of daily metabolisable energy (ME) intake and weekly plasma concentrations of E1 and E2 were determined for each cat for each experimental period: pre-OX adaptation, post-OX E2 injection, post-OX vehicle injection, post-OX unrestricted diet presentation, post-OX-restricted diet presentation. The significances of between-experimental period differences in these variables along with body weight and body fat percentage were determined using paired t tests. Linear regression analysis was used to determine the significance of correlations between variable observations within an experimental period. Statistical analyses were conducted with SAS for Windows software, version 9.1.3 (SAS Institute, Cary, NC, USA).
Results and discussion
Neutering and oestradiol injection
The mean ME intakes among the cats after OX, during the period of vehicle injections, were not significantly different from those observed during the pre-OX period (Table 1). An increase in ME intake after OX was expected(Reference Backus, Cave and Keisler9). The cause for the lacking change in food intake is unknown. The post-OX limiting of diet amount to 110 % of the pre-OX ad libitum intake is probably contributing. In order to control for temporal variation in environmental factors that might affect food intake, studies showing that OX increases food intake in male cats compare simultaneous observations between OX and sexually intact controls(Reference Kanchuk, Backus and Calvert1).
(Mean values with their standard errors, n 6)
E2, estradiol; ND, not determined; E1, estrone.
* Mean value was greater than the restricted values (P = 0·01).
† Tended to be greater than the restricted values (P = 0·09).
‡ Less than the pre-OX values (P = 0·04).
§ Tended to be greater than the vehicle value (P = 0·02).
During the post-OX period, food intakes among all cats during injections of E2 were not significantly different compared with those during vehicle injections (Table 1). However, in five of the six cats, food intakes during E2 injections relative to vehicle injections were less, a mean of 14 (se 3) % less. For reference, OX of cats results in mean increases of food intake of 12–15 %(Reference Kanchuk, Backus and Calvert1). Body weights of cats not responding to E2 injections (7·1 kg) were substantially greater than those of the other cats (4·3–5·3 kg); hence, the E2 dosage for cats was the lowest used (0·07 v. 0·9–0·12 μg/kg).
Plasma E2 concentrations during the post-OX period when vehicle injections were given were lower (P = 0·04) than the concentrations before OX (Table 1). When E2 injections were given during the post-OX period, plasma E2 concentrations at 4 h after injections were greater (P = 0·02) relative to those when the vehicle injections were given and not significantly different from pre-OX plasma E2 concentrations. The means of plasma E2 concentrations after a single E2 injection were 190, 149, 124, 133 and 160 % of baseline concentrations at 2, 4, 8, 16 and 24 h, respectively. The time-weighted arithmetic mean of plasma E2 concentrations over the 24 h period was 147 % of baseline. Hence, the plasma E2 concentrations at the 4 h sampling time appeared to be representative of the elevation of E2 in plasma following E2 injections.
Plasma E1 concentrations before OX were not significantly different after OX or E2 administration (Table 1). These concentrations were approximately five- to ten-fold greater than E2 concentrations, a condition observed in other species(Reference Yasui, Yamada and Kinoshita8).
Post-neutering weight gain
The mean daily ME intake during continuous presentation of the diet for 8 weeks was substantially greater (P = 0·01) than that when diet presentation was restricted to 110 % of pre-OX intake (Table 1). At the end of the period of continuous diet presentation, great variation among the cats was observed in body weights (8·2–5·0 kg) and fat percentages (19–32 %). Nonetheless, the body weights were greater (P = 0·01) and body fat percentages tended to be greater (P = 0·09) than those at the end of the periods of restricted diet access. Additionally, the body fat percentages tended to be positively correlated (r 0·78, P = 0·07) with mean daily ME intakes during the periods of unrestricted diet presentation.
While food intake and adiposity appeared affected by the degree of diet presentation, the abundance of circulating oestrogens appeared unaffected. The E1 and E2 concentrations in plasma were not significantly different between the restricted and unrestricted periods of diet presentation (Table 1). They were also not significantly correlated with body fat percentage at the end of the periods of unrestricted diet presentation. Hence, expansion of adipose mass following OX does not appear to substantially affect circulating concentrations of E1 and E2 in cats.
In conclusion, E1 and a portion of E2 in the plasma of male cats appear to originate from extragonadal sources. The plasma E2 concentrations appear to be reduced by about half following OX. Exogenous E2 at a dose reported to reduce food intake appears to restore pre-OX plasma E2 concentrations.
Acknowledgements
There are no conflicts of interests known to the author. The present study received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
