Neonatal Kisspeptin is Steroid-Independently Required for Defeminisation and Peripubertal Kisspeptin-Induced Testosterone is Required for Masculinisation of the Brain: A Behavioural Study Using Kiss1 Knockout Rats

Rodents show apparent sex differences in their sexual behaviours. The present study used Kiss1 knockout (KO) rats to evaluate the role of kisspeptin in the defeminisation/masculinisation of the brain mechanism that controls sexual behaviours. Castrated adult Kiss1 KO males treated with testosterone showed no male sexual behaviours but demonstrated the oestrogen-induced lordosis behaviours found in wild-type females. The sizes of some of the sexual dimorphic nuclei of Kiss1 KO male rats are similar to those of females. Plasma testosterone levels at embryonic day 18 and postnatal day 0 (PND0) in Kiss1 KO males were high, similar to wild-type males, indicating that perinatal testosterone is secreted in a kisspeptin-independent manner. Long-term exposure to testosterone from peripubertal to adult periods restored mounts and intromissions in KO males, suggesting that kisspeptin-dependent peripubertal testosterone secretion is required to masculinise the brain mechanism. This long-term testosterone treatment failed to abolish lordosis behaviours in KO males, whereas kisspeptin replacement at PND0 reduced lordosis quotients in Kiss1 KO males but not in KO females. These results suggest that kisspeptin itself is required to defeminise behaviour in the perinatal period, in cooperation with testosterone. Oestradiol benzoate treatment at PND0 suppressed lordosis quotients in Kiss1 KO rats, indicating that the mechanisms downstream of oestradiol work properly in the absence of kisspeptin. There was no significant difference in aromatase gene expression in the whole hypothalamus between Kiss1 KO and wild-type male rats at PND0. Taken together, the present study demonstrates that both perinatal kisspeptin and kisspeptin-independent testosterone are required for defeminisation of the brain, whereas kisspeptin-dependent testosterone during peripuberty to adulthood is needed for masculinisation of the brain in male rats.

Severe or life-threatening asthma exacerbation: patient heterogeneity identified by cluster analysis

BACKGROUND

Severe or life-threatening asthma exacerbation is one of the worst outcomes of asthma because of the risk of death. To date, few studies have explored the potential heterogeneity of this condition.

OBJECTIVES

To examine the clinical characteristics and heterogeneity of patients with severe or life-threatening asthma exacerbation.

METHODS

This was a multicentre, prospective study of patients with severe or life-threatening asthma exacerbation and pulse oxygen saturation < 90% who were admitted to 17 institutions across Japan. Cluster analysis was performed using variables from patient- and physician-orientated structured questionnaires.

RESULTS

Analysis of data from 175 patients with severe or life-threatening asthma exacerbation revealed five distinct clusters. Cluster 1 (n = 27) was younger-onset asthma with severe symptoms at baseline, including limitation of activities, a higher frequency of treatment with oral corticosteroids and short-acting beta-agonists, and a higher frequency of asthma hospitalizations in the past year. Cluster 2 (n = 35) was predominantly composed of elderly females, with the highest frequency of comorbid, chronic hyperplastic rhinosinusitis/nasal polyposis, and a long disease duration. Cluster 3 (n = 40) was allergic asthma without inhaled corticosteroid use at baseline. Patients in this cluster had a higher frequency of atopy, including allergic rhinitis and furred pet hypersensitivity, and a better prognosis during hospitalization compared with the other clusters. Cluster 4 (n = 34) was characterized by elderly males with concomitant chronic obstructive pulmonary disease (COPD). Although cluster 5 (n = 39) had very mild symptoms at baseline according to the patient questionnaires, 41% had previously been hospitalized for asthma.

CONCLUSIONS & CLINICAL RELEVANCE

This study demonstrated that significant heterogeneity exists among patients with severe or life-threatening asthma exacerbation. Differences were observed in the severity of asthma symptoms and use of inhaled corticosteroids at baseline, and the presence of comorbid COPD. These findings may contribute to a deeper understanding and better management of this patient population.

Lack of pulse and surge modes and glutamatergic stimulation of luteinising hormone release in Kiss1 knockout rats

Kisspeptin, encoded by the Kiss1 gene, has attracted attention as a key candidate neuropeptide in controlling puberty and reproduction via regulation of gonadotrophin-releasing hormone (GnRH) secretion in mammals. Pioneer studies with Kiss1 or its cognate receptor Gpr54 knockout (KO) mice showed the indispensable role of kisspeptin-GPR54 signalling in the control of animal reproduction, although detailed analyses of gonadotrophin secretion, especially pulsatile and surge-mode of luteinising hormone (LH) secretion, were limited. Thus, in the present study, we have generated Kiss1 KO rats aiming to evaluate a key role of kisspeptin in governing reproduction via pulse and surge modes of GnRH/LH secretion. Kiss1 KO male and female rats showed a complete suppression of pulsatile LH secretion, which is responsible for folliculogenesis and spermatogenesis, and an absence of puberty and atrophic gonads. Kiss1 KO female rats showed no spontaneous LH/follicle-stimulating hormone surge and an oestrogen-induced LH surge, suggesting that the GnRH surge generation system, which is responsible for ovulation, does not function without kisspeptin. Furthermore, challenge of major stimulatory neurotransmitters, such as monosodium glutamate, NMDA and norepinephrine, failed to stimulate LH secretion in Kiss1 KO rats, albeit they stimulated LH release in wild-type controls. Taken together, the results of the present study confirm that kisspeptin plays an indispensable role in generating two modes (pulse and surge) of GnRH/gonadotrophin secretion to regulate puberty onset and normal reproductive performance. In addition, the present study suggests that kisspeptin neurones play a critical role as a hub integrating major stimulatory neural inputs to GnRH neurones, using newly established Kiss1 KO rats, which serve as a useful model for detailed analysis of hormonal profiles.

202 EFFECT OF LEUKEMIA INHIBITORY FACTOR AND FORSKOLIN ON ESTABLISHMENT OF RAT EMBRYONIC STEM CELL LINES

Rat embryonic stem (ES) cell lines can be established in culture medium containing inhibitors for glycogen synthase kinase 3 (GSK3) and mitogen-activated protein kinase (MEK). We confirmed reproducibility of the 2i culture system in establishing rat ES cell lines (Hirabayashi et al. 2010 Mol. Reprod. Dev. 77, 94) and the likelihood of successful germline transmission (genuine) of rat ES cell lines established in leukemia inhibitory factor (LIF)- and forskolin (FK)-supplemented 2i medium (Hirabayashi et al. 2013 Transgenic Res. 22, 411–416). This study was designed to investigate whether LIF and/or FK supplemented to the 2i medium support establishment of germline-competent rat ES cell lines. E4.5 blastocysts were recovered from BLK rat females, and zona-free embryos were plated on mitomycin-treated mouse embryonic fibroblasts in N2B27 medium containing 1 mM MEK inhibitor PD0325901 and 3 mM GSK3 inhibitor CHIR99021, with rat 1000 U mL–1 LIF and/or 10 μM FK. Outgrowth rate of the blastocysts after 1 wk culture and establishment efficiency of ES cell lines after third passage were analyzed by Fisher's exact probability test. Arcsin-transformed percentage data on full-term development of ES cell-injected blastocysts, chimeric rat production, and germline-competent chimeras were analyzed by Fisher's least significant difference test after one-way ANOVA. Because of the higher outgrowth rates of blastocysts, supplementation of rat LIF, FK, or both contributed to the higher (P < 0.05) establishment efficiency of ES cell lines in BLK rat strain (76% to 92% v. 50% in LIF/FK-free 2i medium). Neither efficiency of producing chimeric rats (14% to 39% of blastocysts injected) nor germline transmission competency of the chimeric rats (67% to 100% of cell lines analyzed) was influenced by the pre-treatment of ES cell lines. When the LIF/FK-supplemented 2i medium was used, rat strain for blastocyst donor such as F344 or WI was a possible factor negatively influencing the establishment efficiency of ES cell lines. Once ES cell lines were established, however, all of them (9/9 in overall) were found to be germline-competent by progeny test of chimeric rats. In conclusion, both LIF and FK are not essential, but can play a beneficial role, for the establishment of genuine rat ES cell lines. This work was supported by a grant-in-aid for basic research from Japan Society for the Promotion of Science (No. 25290037; to M.H.).

60 EFFECT OF CAKE COLLAPSE ON THE INTEGRITY OF FREEZE-DRIED BULL SPERMATOZOA

During freezing, a solution changes into an amorphous phase at the glass transition temperature of the maximally freeze-concentrated phase (T′g). The solution exhibits a cake-like porous structure under the optimal freeze-drying process. However, if the product temperature is higher than theT′g during the drying phase, the glassy material will undergo viscous flow, resulting in loss of the porous structure. This is defined as the collapse phenomenon and may be related to instability of the freeze-dried products. The purpose of the present study was to investigate the effect of cake collapse on freeze-dried bull spermatozoa. One-way ANOVA was used for comparison of T′g, DNA damage, and blastocyst yield. When the ANOVA was significant, differences among means were analysed by a Tukey test. In Experiment 1, factors affecting the T′g were investigated. Using differential scanning calorimetry, theT′g of an EGTA buffer (10 mM TRIS-HCl, 50 mM EGTA, and 50 mM NaCl, pH8.0) that has been conventionally used for sperm freeze-drying was determined to be –45.0°C. Modification of the EGTA buffer composition by complete removal of NaCl and addition of 0.5 M trehalose (referred to hereafter as mEGTA buffer) resulted in an increase in theT′g up to –27.7°C. The T′g of the mEGTA buffer cooled by direct immersing into liquid nitrogen (–29.4°C) was slightly lower (P < 0.05) than that cooled slowly at 20 and 1°C min–1 (–27.6 and –27.2°C, respectively). In Experiment 2, the integrity of freeze-dried and rehydrated bull spermatozoa was investigated. Spermatozoa from a Japanese Black bull were suspended into mEGTA buffer (3 × 107 cells mL–1), cooled at 20°C min–1, and then processed for drying for 6 h at 0, –15, and –30°C (ALPHA2-4; Martin Christ Gefriertrocknungsanlagen GmbH, Osterode am Harz, Germany). Cakes were collapsed when the sperm suspension was dehydrated either at 0 or –15°C. In vitro-matured bovine oocytes were injected with rehydrated sperm, chemically activated (5 μM ionomycin, 7% ethanol, and 2 mM 6-DMAP), and then cultured for 8 days. Blastocyst yields after injection of sperm dried at 0 and –15°C, calculated from cleaved oocytes, were significantly lower than that of sperm dried at –30°C (0.7–3.7% v. 14.2%; P < 0.05). The level of DNA damage, assessed by the alkaline comet assay, was not different between the sperm populations dried at 0 and –30°C. Transmission electron microscopic observation revealed that the sperm membrane dried at 0°C was more damaged compared with that dried at –30°C (P < 0.05; chi-squared test with Bonferroni correction). In conclusion, incidence of collapse in freeze-dried cake may be a detrimental factor for maintenance of sperm integrity after freeze-drying, and can be inhibited by controlling the T′g of the buffer and drying phase temperature. H. Hara is Research Fellow of the Japan Society for the Promotion of Science (JSPS). This work was supported by a grant-in-aid for basic research from JSPS (no. 24580407) to S. Hochi.

High incidence of multiple aster formation in vitrified-warmed bovine oocytes after in vitro fertilization

In vitro-matured bovine oocytes do not tolerate vitrification as well as mature murine or human oocytes. Delayed first cleavage in vitrified and in vitro-fertilized bovine oocytes may be responsible for the decreased yield of blastocysts in vitro. Because formation of sperm-aster and the subsequent assembly of microtubule network play an important role for migration and fusion of both pronuclei, aster formation in vitrified-warmed oocytes was analyzed by confocal laser-scanning microscopy. At 10 h post-insemination (hpi), proportions of oocytes fertilized normally were comparable between the vitrified and fresh control groups (67 and 70%, respectively). Proportions of oocytes that exhibited microtubule assembly were similar between the two groups (95% each), but the proportion of oocytes with multiple asters was higher in the vitrified group when compared with the fresh control group (68 vs 29%, P < 0.05). Both migration and development of two pronuclei were adversely affected by multiple aster formation. In the next experiment, multiple asters observed in 5.5 vs 8 hpi pronuclear zygotes were located near the male pronucleus, suggesting that those multiple asters were not the cytoplasmic asters of maternal origin. In conclusion, multiple aster formation frequently observed in vitrified-warmed bovine oocytes may be related to loss of ooplasmic function responsible for normal microtubule assembly from the sperm-aster.

Stimulatory effect of Rho-associated coiled-coil kinase (ROCK) inhibitor on revivability of in vitro-produced bovine blastocysts after vitrification

Inhibition of Rho-associated coiled-coil kinase (ROCK) activity promoted recovery and growth of frozen-thawed human embryonic stem cells. The primary objective was to determine if a ROCK inhibitor (Y-27632) in post-thaw culture medium improved revivability of vitrified IVP bovine blastocysts. Expanding or expanded blastocysts (7 d after IVF) were vitrified (minimum volume cooling procedure, using a Cryotop) in 15% ethylene glycol, 15% DMSO and 0.5M sucrose. When post-warm blastocysts were cultured in mSOF medium, survival rate (re-expansion of blastocoel at 24h of culture) was improved (P<0.05) by the addition of 10 microM Y-27632 (94.9+/-2.4%, mean+/-SEM) compared to a control (78.0+/-6.0%). Conversely, after 48 h of culture, there were no significant differences in hatching rate (62.8+/-11.1 vs. 59.6+/-9.4%) and mean total cell number (135.2+/-13.1 vs. 146.7+/-13.3). In non-vitrified IVP bovine blastocysts, the hatching rate on Day 9 was improved by Y-27632 (91.7+/-3.8 vs. 54.7+/-8.9%, P<0.05), with no difference in mean total cell number of blastocysts (230.0+/-23.0 vs. 191.2+/-22.2, P=0.23). In an additional experiment, Y-27632 was added to culture medium on either Day 0, Day 2, or Day 4 (and remained present until Day 8), resulting in no improvement in blastocyst yield compared to a control group (7.5+/-2.1, 31.4+/-2.3, 36.2+/-3.2, and 28.6+/-6.9%, respectively). In conclusion, adding a ROCK inhibitor to post-thaw culture medium improved revivability of IVP bovine blastocysts after vitrification and warming.

Fluorescent labeling of both GABAergic and glycinergic neurons in vesicular GABA transporter (VGAT)-venus transgenic mouse

Inhibitory neurons play important roles in a number of brain functions. They are composed of GABAergic neurons and glycinergic neurons, and vesicular GABA transporter (VGAT) is specifically expressed in these neurons. Since the inhibitory neurons are scattered around in the CNS, it is difficult to identify these cells in living brain preparations. The glutamate decarboxylase (GAD) 67-GFP knock-in mouse has been widely used for the identification of GABAergic neurons, but their GAD67 expression was decreased compared to the wild-type mice. To overcome such a problem and to highlight the function and morphology of inhibitory neurons, we generated four lines of VGAT-Venus transgenic mice (lines #04, #29, #39 and #49) expressing Venus fluorescent protein under the control of mouse VGAT promoter. We found higher expression level of Venus transcripts and proteins as well as brighter fluorescent signal in line #39 mouse brains, compared to brains of other lines examined. By Western blots and spectrofluorometric measurements of forebrain, the line #39 mouse showed stronger GFP immunoreactivity and brighter fluorescent intensity than the GAD67-GFP knock-in mouse. In addition, Venus was present not only in somata, but also in neurites in the line #39 mouse by histological studies. In situ hybridization analysis showed that the expression pattern of Venus in the line #39 mouse was similar to that of endogenous VGAT. Double immunostaining analysis in line #39 mouse showed that Venus-expressing cells are primarily immunoreactive for GABA in cerebral cortex, hippocampus and cerebellar cortex and for GABA or glycine in dorsal cochlear nucleus. These results demonstrate that the VGAT-Venus line #39 mouse should be useful for studies on function and morphology of inhibitory neurons in the CNS.

269 DEMETHYLATION DYNAMICS OF PATERNAL GENOMES IN PRONUCLEAR BOVINE ZYGOTES PRODUCED BY IN VITRO FERTILIZATION AND INTRACYTOPLASMIC SPERM INJECTION WITH NONDRIED OR FREEZE-DRIED SPERMATOZOA

Epigenetic reprogramming in early preimplantation embryos, which refers to erasing and remodeling epigenetic marks, including DNA methylation, is essential for differentiation and development. In many species, the paternal genome is subjected to genome-wide active demethylation before DNA replication commences, whereas the maternal genome maintains its methylation status until being demethylated passively during the subsequent cleavage divisions. This study was designed to investigate the dynamics of paternal genome demethylation in pronuclear-stage bovine zygotes produced by conventional IVF or intracytoplasmic sperm injection (ICSI) using conventionally frozen, nondried and freeze-dried (FD) bull sperm stored at either +4 or –196°C for 1 year. Zygotes were fixed and immunostained using anti-5 methyl cytosin (Calbiochem, San Diego, CA, USA) at 8, 10, 14 and 18 h post IVF (hpi) and at 6 and 12 h post ICSI (hpic). After digital optical sections of the zygotes had been collected by a Bio-Rad confocal laser scanning microscope equipped with a krypton/argon ion laser, the fluorescence emitted by each pronucleus was quantitatively analyzed by Image-J software. The relative methylation of the male pronucleus to the female pronucleus in each zygote (RM) was calculated. Differences among groups regarding proportions of zygotes showing different levels of RM were analyzed using Fisher’s exact probability test, whereas differences among groups regarding the overall average of RM were analyzed using Student’s t-test. In conventional IVF, the overall average RM significantly decreased (P < 0.05) from 0.92 at 8 hpi to 0.69 at 10 hpi with no additional significant decrease at 14 and 18 hpi (0.67 and 0.64, respectively). This decrease was accompanied by significantly higher proportions of zygotes showing RM < 0.60 (45.5, 37.5, and 38.2% at 10, 14, and 18 hpi, respectively) compared with 3.7% at 8 hpi. Because spermatozoa take about 2 h to penetrate into oocytes in a bovine IVF system, zygotes at 6 and 12 hpic may be developmentally comparable with those at 8 and 14 hpi. The overall averages of RM in ICSI-derived zygotes (0.79 and 0.66 at 6 and 12 hpic, respectively) were similar to those in corresponding IVF-derived zygotes (8 and 14 hpi), but a significantly higher proportion of 6 hpic zygotes (37.8%) than 8 hpi zygotes (3.7%) showed RM < 0.60. Surprisingly, the proportions of 12 hpic zygotes produced with FD sperm showing RM < 0.60 (60.6 and 62.4% for +4 and –196°C storage, respectively) were significantly higher than the proportions of those produced with nondried sperm (39.4%). These findings indicate that demethylation of the bovine paternal genome occurred early after IVF and ICSI, and that the freeze-drying of the paternal genome resulted in a higher demethylation level.

Impact of stent design on intra-aneurysmal flow. A computer simulation study

SUMMARY

In addition to providing a skeleton for vessel reconstruction, stent implantation as used for cerebral aneurysm treatment can induce flow redirection, thus reducing vortical flow velocities within the aneurysm cavity. Further, stent characteristics such as strut size, porosity and cell shape influence the changes in intra-aneurysmal flow by analog simulations. The purpose of this computer simulation study was to visualize the flow pattern over the entire neck area of a side wall aneurysm while changing the stent parameters. A 3-D computer model aneurysm was constructed to have a parent artery of 5 mm diameter and an aneurysm of 10 mm diameter. The distance between the midline of main artery and center point of the aneurysm was 6.8 mm, providing a neck length of 5 mm, a width of 3.6 mm, and a neck area of 14 mm 2. The simulations were carried out with a Finite Element Method based flow simulation package. The incompressible Navier-Stokes equation was solved for a steady flow with a mean speed of 290 mm/s, steady viscosity of 3.83 cp, and density of 1.0 g/cm3. Two parallel stent struts (dimensions: 100 mum m 100 mum m 2.0 mm) were introduced into the plane of the aneurysm neck. The fraction of the aneurysm neck cross-section occupied by the stent was 2.83% in all cases. The velocity distribution through the neck of the aneurysm was calculated for three different choices of separation between the struts for each of two orientations of the struts (parallel and perpendicular) relative to the vessel axis. The flow pattern in the aneurysm was composed of an inflow zone at the distal neck and of an outflow zone at the proximal neck. The placement of stent struts at the aneurysm neck resulted in a decrease in the mean speed in the aneurysm. The degree of reduction and the distribution of flow through the neck did depend on the orientation of the stent struts. The struts, when placed parallel or perpendicular to the parent vessel axis affected the mean speed through the aneurysm neck differently.

The ability of whale haploid spermatogenic cells to induce calcium oscillations and its relevance to oocyte activation

Interspecies microinsemination assay was applied to examine the ability of minke whale haploid spermatogenic cells to induce Ca2+oscillations and oocyte activation. Populations of round spermatids (RS), early-stage elongating spermatids (e-ES), late-stage elongating spermatids (1-ES) and testicular spermatozoa (TS) were cryopreserved in the presence of 7.5% glycerol on board ship in the Antarctic Ocean. Repetitive increases of intracellular Ca2+concentration occurred in 0, 65, 81 and 96% of BDF1 mouse oocytes injected with the postthaw RS, e-ES, 1-ES and TS, respectively. A normal pattern of the Ca2+oscillations was observed in 26–47% of the responding oocytes. Most oocytes that exhibited Ca2+oscillations, regardless of the oscillation pattern, resumed meiosis (83–94%). These results indicate that whale spermatogenic cells acquire SOAF activity, which is closely related to their Ca2+oscillation-inducing ability at the relatively early stage of spermiogenesis.

Protease activity higher in postmortem water buffalo meat than Brahman beef

We previously demonstrated that postmortem water buffalo meat had higher tenderness than Brahman beef. In order to explain this difference in tenderness, the objective of the current study was to investigate the protease activity in these two meats. Five female crossbred water buffalo (Philippine Carabao×Bulgarian Murrah) and five female crossbred cattle (Brahman×Philippine Native) were slaughtered at 30months of age, followed by immediate sampling of Longissimus thoracis muscle for measurement of protease activity. Results showed that buffalo meat had significantly higher protease activity compared to beef (P<0.05). Furthermore, calpain inhibitor 1, a specific inhibitor of calpains 1 and 2, was the most effective inhibitor of protease activity. There was no difference in calpastatin activity, and no major differences were observed in calpains 1, 2, and calpastatin expression by Western blotting. This study suggests that higher calpain activity in early postmortem buffalo meat was responsible for the increased tenderness of water buffalo meat compared to beef.

69 APPLICATION OF BOVINE AMNIOTIC CELLS FOR PRENATAL GENETIC DIAGNOSIS AND NUCLEAR TRANSFER

Amniotic fluid includes many cells derived from the fetus called 'amniotic cells'. Although these amniotic cells may have much potential as a reproduction or breeding source of the animals, there has been limited study of the potential applications of these cells. We examined the potential of bovine amniotic cells for biotechnological use. Bovine amniotic cells separated from amniotic fluid obtained from a slaughterhouse were prepared for use in all experiments. First, to examine the culture condition of amniotic cells, the cells were cultured in various culture media. Cytologic normality of the cultured cells was analyzed by chromosomal examination and Papanicolaou examination. Second, to examine the potential of cultured amniotic cells for prenatal genetic diagnosis, the cells were used for sexing by PCR. The coincidence of the results with the gender of the fetus from which the cells were derived was examined. Third, we used the cultured cells as donor cells for nuclear transfer, and examined the developmental ability of reconstructed embryos. The normality of the blastocysts derived from the reconstructed embryos was examined by chromosomal examination and transplantation to the recipient heifer. Bovine amniotic cells were cultured successfully in Amnio-max C-100� (GIBCO, Grand Island, NY, USA), which is marketed as culture medium for human amniotic cells. In all cases, the sex of cultured amniotic cells analyzed by PCR was coincident with that of the fetus from which the amniotic cells were derived. The frequencies of cleavage and development to the blastocyst stage of embryos reconstructed from amniotic cells were the same as those of fetal fibroblasts. There were no differences in the normality of chromosomal number between blastocysts derived from amniotic cells and fetal fibroblasts. A blastocyst derived from amniotic cells developed into a fetus after transplantation. DNA microsatellite analysis of the fetus at Day 64 was coincident with that of the fetus from which the amniotic cells were derived. These results indicate that bovine amniotic cells can be successfully cultured in vitro, and the cultured cells precisely reflect the genetic information of the fetus from which the cells were derived. The cultured cells also have developmental ability as donor cells for nuclear transfer. Amniotic cells may have the potential for effective reproduction and breeding using genetic and biotechnological sources.

Difference in tenderness and pH decline between water buffalo meat and beef during postmortem aging

The objective of this research was to determine the difference in tenderness and some characteristics of water buffalo meat and beef during postmortem aging. Five female crossbred water-buffalo (Philippine Carabao×Bulgarian Murrah) and five female crossbred cattle (Brahman×Philippine Native), were finished on the same diet for 6 months and slaughtered at 30 months of age. The muscle pH was measured at 40min, 3h, 7h, 24h, and 48h postmortem. Longissimus thoracis (LT) and semimembranosus (SM) muscles were excised at 2d postmortem, and shear force was measured at 2, 4, 7, and 14d postmortem. Glycogen and lactate concentrations were determined from 0, 2, and 4d LT samples, and myosin heavy chain type of buffalo and cattle LT was determined by ELISA methods. Myofibrillar protein degradation was also observed by SDS-PAGE and Western blotting of fast-type troponin T. Results showed that the buffalo meat had significantly lower shear force values compared to beef for LT and SM muscles, which was supported by a difference in troponin T degradation. Postmortem pH decline of buffalo meat was significantly slower than that of beef, which was confirmed by lactic acid concentrations, but was not explained by glycogen content. In addition, there was no significant difference in the ratio of slow to fast type muscle fibers in buffalo and cattle, indicating that myosin heavy chain type was not responsible for the difference in pH decline and tenderness between the buffalo meat and beef. This study demonstrated that the tenderness of water buffalo meat was superior to that of Brahman beef, which may have been due to the difference in pH decline and the subsequent effect on muscle protease activity.