Effect of hypothyroidism on the lipid composition and fluidity of rat colonic apical plasma membranes

The Effects of Altered Membrane Cholesterol Levels on Sodium Pump Activity in Subclinical Hypothyroidism

BACKGROUND

Metabolic dysfunctions characteristic of overt hypothyroidism (OH) start at the early stage of subclinical hypothyroidism (SCH). Na⁺/K⁺-ATPase (the sodium pump) is a transmembrane enzyme that plays a vital role in cellular activities in combination with membrane lipids. We evaluated the effects of early changes in thyroid hormone and membrane cholesterol on sodium pump activity in SCH and OH patients.

METHODS

In 32 SCH patients, 35 OH patients, and 34 euthyroid patients, sodium pump activity and cholesterol levels in red blood cell membranes were measured. Serum thyroxine (T₄) and thyroid stimulating hormone (TSH) levels were measured using enzyme-linked immunosorbent assays. Differences in their mean values were analysed using post hoc analysis of variance. We assessed the dependence of the sodium pump on other metabolites by multiple regression analysis.

RESULTS

Sodium pump activity and membrane cholesterol were lower in both hypothyroid groups than in control group, OH group exhibiting lower values than SCH group. In SCH group, sodium pump activity showed a significant direct dependence on membrane cholesterol with an inverse relationship with serum TSH levels. In OH group, sodium pump activity depended directly on membrane cholesterol and serum T₄ levels. No dependence on serum cholesterol was observed in either case.

CONCLUSION

Despite the presence of elevated serum cholesterol in hypothyroidism, membrane cholesterol contributed significantly to maintain sodium pump activity in the cells. A critical reduction in membrane cholesterol levels heralds compromised enzyme activity, even in the early stage of hypothyroidism, and this can be predicted by elevated TSH levels alone, without any evident clinical manifestations.

Alterations in osmotic fragility of the red blood cells in hypo- and hyperthyroid patients

BACKGROUND

Changes in concentration of thyroid hormones can affect Na+-K+-ATPase number and activity and phospholipid composition of the cell membranes leading to changes in the surface to volume ratio and strength of membrane.

AIM

In this study, the osmotic fragility of the red blood cells from non-treated hypo- and hyperthyroid patients was compared to that of control subjects.

MATERIAL/SUBJECTS AND METHODS

After 3 washings with normal saline, red blood cells were placed in varying concentrations of sodium chloride (Na- Cl) (0-0.9%) and fragility was assessed with colorimetric method; to do this, after the incubation period, tubes were centrifuged and the optical density of the tubes was measured. Hemolysis percentage in tubes was calculated based on 100% hemolysis in the tubes containing no NaCl (0%).

RESULTS

Osmotic fragility of the cells from hyperthyroid patients in 0.45% NaCl was significantly lower than control subjects (74.6%+/-30.2 vs 93.8%+/-9.1, p<0.01). The osmotic fragility of red blood cells in 0.5% concentration of sodium chloride in hyperthyroid patients was significantly lower compared to that of controls (27.8%+/-26.0 vs 63.5%+/-27.5, p<0.001). No significant difference was observed between the osmotic fragility of the hypothyroid patients compared with control subjects.

CONCLUSIONS

Alteration in osmotic fragility is seen in patients with hyperthyroidism; however, anemia reported in hypo- or hyperthyroid patients is not due to high osmotic fragility of red blood cells and other causes need to be investigated.

Thyroid hormone effects on mitochondrial energetics

Thyroid hormones are the major endocrine regulators of metabolic rate, and their hypermetabolic effects are widely recognized. The cellular mechanisms underlying these metabolic effects have been the subject of much research. Thyroid hormone status has a profound impact on mitochondria, the organelles responsible for the majority of cellular adenosine triphosphate (ATP) production. However, mechanisms are not well understood. We review the effects of thyroid hormones on mitochondrial energetics and principally oxidative phosphorylation. Genomic and nongenomic mechanisms have been studied. Through the former, thyroid hormones stimulate mitochondriogenesis and thereby augment cellular oxidative capacity. Thyroid hormones induce substantial modifications in mitochondrial inner membrane protein and lipid compositions. Results are consistent with the idea that thyroid hormones activate the uncoupling of oxidative phosphorylation through various mechanisms involving inner membrane proteins and lipids. Increased uncoupling appears to be responsible for some of the hypermetabolic effects of thyroid hormones. ATP synthesis and turnover reactions are also affected. There appear to be complex relationships between mitochondrial proton leak mechanisms, reactive oxygen species production, and thyroid status. As the majority of studies have focused on the effects of thyroid status on rat liver preparations, there is still a need to address fundamental questions regarding thyroid hormone effects in other tissues and species.

Lipid composition effect on permeability across PAMPA

The parallel artificial membrane permeability assay (PAMPA) system has promise to rapidly screen drug candidate passive permeability, but has been poorly described in terms of its lipid membrane structure and function. The objective was to investigate the role of PAMPA lipid composition on the permeability of five model compounds. PAMPA was used and employed individual phospholipids that varied in phosphate head group and acyl chain unsaturation. Transport of benzoic acid, taurocholic acid, metoprolol, sucrose, and mannitol was measured. Membrane fluidity was assessed by 1,3-diphenylhexatriene fluorescence anisotropy. Results indicate that compound permeability across PAMPA differed in their sensitivity to membrane lipid composition, where compounds with appreciable permeability (i.e. at least 0.2 x 10(-6)cm/s) were possibly sensitive to membrane fluidity and apparent ion pair effects. Benzoic acid permeability ranged 51-fold across membrane types, suggesting acyl chain effect on membrane fluidity. Mannitol, sucrose, and taurocholic acid permeabilities were low and independent of lipid composition. Metoprolol permeability ranged 17-fold and exhibited a markedly high permeability across 1,2-dioleoyl-sn-glycero-3-[phospho-L-serine] due to apparent ion pair-facilitated transport. Compound permeability was lowest across the phosphatidylcholines, which is consistent with phosphatidylcholine exhibiting relatively high membrane rigidity. In contrast to results from phosphatidylethanolamines and phosphatidylserines, acyl chain unsaturation had no effect on permeability across phosphatidylcholines. In conclusion, while much remains unknown about PAMPA structure and subsequent PAMPA permeability, results here from five solutes suggest that, for solutes with appreciable permeability, lipid composition modulated drug permeability through possible membrane fluidity and apparent ion pair influences.

Ion pair-mediated transport of metoprolol across a three lipid-component PAMPA system

Parallel Artificial Membrane Permeability Assay (PAMPA) is a method to screen drug candidates for membrane permeability. The objective was to characterize the transport of a model weak base, metoprolol, across a three lipid-component PAMPA system (denoted A-PAMPA, for anionic-PAMPA) and challenge ion pairing as a mechanism for metoprolol transport. A-PAMPA was designed to mimic the lipid composition of the enterocyte's plasma membrane and included 1,2-dioleoyl-sn-glycero-3-[phospho-l-serine] (PS18:1) as an anionic lipid-component. Metoprolol flux was measured across A-PAMPA, as well as across three other PAMPA systems. Permeability studies were conducted under various conditions, with varying pH, ionic strength, and presence/absence of competing cations. Permeabilities of mannitol and benzoic acid, as model neutral and anionic solutes, were also measured. PAMPA membrane fluidity was inferred from anisotropy measurements in liposomes. Ion pairing between metoprolol and PS18:1 was assessed via NMR. Metoprolol transport across A-PAMPA was dominated by an ion pair-mediated mechanism (i.e. metoprolol-PS18:1 complex), rather than a membrane fluidity-mediated mechanism. Compared to other PAMPA systems, metoprolol permeability across A-PAMPA and PS18:1 was high. Permeability and anisotropy values suggested PS18:1 selectively facilitated metoprolol transport, while neutral lipid did not. Additional studies supporting ion pairing of metoprolol across A-PAMPA showed that a) metoprolol transport was self-inhibited across A-PAMPA but not across neutral lipid PAMPA; b) competing cations reduced metoprolol permeability across A-PAMPA but not across neutral lipid PAMPA; and c) NMR spectrum of a mixture of metoprolol and PS18:1 showed a broadening of some metoprolol peaks, presumably due to metoprolol interaction with anionic lipid. Metoprolol transport across a three lipid-component PAMPA system that contained anionic lipid was facilitated by apparent ion pairing.

Thyroid hormones increase Na+–Pi co-transport activity in intestinal brush border membrane: Role of membrane lipid composition and fluidity

In the present study, we documented the promising role of thyroid hormones status in animals in modulation of Na+-Pi transport activity in intestinal brush border membrane vesicles (BBMV) which was accompanied with alterations in BBM lipid composition and fluidity. Augmentation of net Pi balance in hyperthyroid (Hyper-T) rats was fraternized with accretion of Pi transport across BBMV isolated from intestine of Hyper-T rats as compared to hypothyroid (Hypo-T) and euthyroid (Eu-T) rats while Na+-Pi transport across BBMV was decreased in Hypo-T rats relative to Eu-T rats. Increment in Na+-Pi transport in intestinal BBMV isolated from Hyper-T rats was manifested as an increase in the maximal velocity (Vmax) of Na+-Pi transport system. Furthermore, BBMV lipid composition profile in intestinal BBM from Hyper-T was altered to that of Hypo-T rats and Eu-T rats. The molar ratio of cholesterol/phospholipids was higher in intestinal BBM from Hypo-T rats. Fluorescence anistropy of diphenyl hexatriene (rDPH) and microviscosity were significantly decreased in the intestinal BBM of Hyper-T rats and decreased in Hypo-T rats as compared to Eu-T rats which corroborated with the alteration in membrane fluidity in response to thyroid hormone status of animals. Therefore, thyroid hormone mediated change in membrane fluidity might play an important role in modulating Na+-Pi transport activity of intestinal BBM.

Thyroid hormones stimulate Na+-Pi transport activity in rat renal brush-border membranes: role of membrane lipid composition and fluidity

Antecedent studies have suggested that lipid composition and fluidity of cellular membranes of various organs are altered in response to thyroid hormone status. To date, the effects of thyroid hormone status on these parameters have not been examined in rat renal apical membrane in regard to sodium-dependent phosphate transport. In the present study, we determined the potential role of alterations in cortical brush-border membrane lipid composition and fluidity in modulation of Na+-Pi transport activity in response to thyroid hormone status. Thyroid hormone status influences the fractional excretion of Pi, which is associated with alteration in renal brush-border membrane phosphate transport. The increment in Na+-Pi transport in renal BBMV isolated from Hyper-T rats is manifested as an increase in the maximal velocity (Vmax) of Na+-Pi transport. Further, the cholesterol content was significantly increased in renal BBM of Hypo-T rats and decreased in Hyper-T rats as compared to the Eu-T rats. The molar ratio of cholesterol/phospholipids was also higher in renal BBM from hypo-T rats. Subsequently, fluorescence anisotropy of diphenyl hexatriene (rDPH) and microviscosity were significantly decreased in the renal BBM of the Hyper-T rats and increased in the Hypo-T rats as compared to Eu-T rats. The result of this study, therefore, suggest that alteration in renal BBM cholesterol, cholesterol/phospholipid molar ratio, and membrane fluidity play an important role in the modulation of renal BBM Na+-Pi transport in response to thyroid hormone status of animals.

Experimental hypothyroidism modifies specific binding of A1 and A2A analogues to adenosine receptors in the rat kidney

1 Binding kinetic studies with the adenosine analogues [3H]CPA (0.250-50 nm) and [3H]CGS21680 (0.1-100 nm) were performed in renal tissue from control (NL) and thyroidectomised (HTX) rats. We propose that the low renal adenosine content reported in hypothyroid rats may induce changes in the density and/or affinity of adenosine receptor, distributed in the cortex (C), outer medulla (OM), and inner medulla (IM) of the kidney. 2 [3H]CPA and [3H]CGS21680 binding saturation isotherms were fitted by nonlinear regression analysis and evaluated by Furchgott's method. These results revealed high (KH) and low (KL) affinity (KD) sites for both compounds. As expected, a heterogeneous pattern was observed for Bmax and KD values. 3 Bound [3H]CPA and [3H]CGS21680 were displaced by increasing concentrations of nonlabelled DPCPX and NECA, respectively, indicating the presence of A1 and A2A adenosine receptors distributed in the renal segments studied. 4 The relative intrinsic efficacy (epsilon) for [3H]CPA and [3H]CGS21680 showed extreme values (far from 1.0), 0.5 in IM NL and 2.70 in IM HTX for [3H]CGS21680. 5 Our results indicate that A2A adenosine receptor is predominant in IM from HTX, but A1 receptors are expressed preferentially in C in NL. 6 We conclude that the changes observed in number, affinity, and epsilon for the A2A receptor in IM from HTX might be responsible from alterations in medullary function, that is, incapacity for urine concentration as observed in the hypothyroid kidney.

Simple method of preparation of rat colonocyte apical membranes using polyethylene glycol precipitation

BACKGROUND

Although a number of methods are available for the preparation of brush border membranes (BBM) from the small intestine, very few studies have been performed on the isolation of colonic apical membranes (CAM). Cation precipitation is one of the methods used for the isolation of these membranes, which can result in altered lipid composition as a result of activation of phospholipase. We have earlier established a method for BBM isolation using polyethylene glycol (PEG) precipitation. In the present study, CAM from isolated colonocytes were prepared using PEG and the isolated membranes were characterized.

METHODS

The CAM were prepared from isolated rat colonocytes using PEG precipitation and differential centrifugation. Purity was assessed by enrichment of the marker enzymes and contamination by other subcellular particles. Lipid composition and sugar components were analyzed in the isolated membranes.

RESULTS

The CAM showed 11-12-fold enrichment in alkaline phosphatase and 9-10-fold enrichment in gamma-glutamyl transpeptidase as compared with the cell homogenate. The final membrane preparation also showed good vesicle formation as seen by electron microscopy. There was very little contamination of basolateral membranes, microsomes or lysosomes in the final membrane preparation. Analysis of sugars indicated high content of fucose and sialic acid as compared with hexoses. Lipid analysis indicated the presence of various neutral and phospholipids and a cholesterol/phospholipid ratio of 0.65 was seen.

CONCLUSION

This study has shown a simple method of CAM preparation using PEG precipitation, which is less time-consuming than other methods and comparatively pure.

Cubilin and megalin expression and their interaction in the rat intestine: effect of thyroidectomy

Cubilin is a 460-kDa multipurpose, multidomain receptor that contains an NH(2)-terminal 110-residue segment followed by 8 epidermal growth factor (EGF)-like repeats and a contiguous stretch (representing nearly 88% of its mass) of 27 CUB (initially found in complement components C1r/C1s, Uegf, and bone morphogenic protein-1) domains. Cubilin binds to intrinsic factor (IF)-cobalamin (cbl, vitamin B(12)) complex and promotes the ileal transport of cbl. The 460-kDa form of cubilin is the predominant form present in the apical brush-border membranes of rat intestine, kidney, and yolk sac, but a 230-kDa form of cubilin is also noted in the intestinal membranes. In thyroidectomized (TDX) rats, levels of intestinal brush-border IF-[(57)Co]-labeled cbl binding, 460-kDa cubilin protein levels and tissue (kidney) accumulation of cbl were reduced by approximately 70%. Immunoblot analysis using cubilin antiserum of intestinal total membranes from TDX rats revealed cubilin fragments with molecular masses of 200 and 300 kDa. Both of these bands, along with the 230-kDa band detected in the total membranes of control rats and unlike the 460-kDa form, failed to react with antiserum to EGF. Mucosal membrane cubilin associated with megalin was reduced from approximately 12% in control to approximately 4% in TDX rats, and this decreased association was not due to altered megalin levels. Thyroxine treatment of TDX rats resulted in reversal of all of these effects, including an increase to nearly 24% of cubilin associated with megalin. In vitro, megalin binding to cubilin occurred with the NH(2)-terminal region that contained the EGF-like repeats and CUB domains 1 and 2 but not with a downstream region that contained CUB domains 2-10. These studies indicate that thyroxine deficiency in rats results in decreased uptake and tissue accumulation of cbl caused mainly by destabilization and deficit of cubilin in the intestinal brush border.

Thyroid hormones modulate zinc transport activity of rat intestinal and renal brush-border membrane

Thyroid hormone status influences the Zn2+ and metallothionein levels in intestine, liver, and kidney. To evaluate the impact of thyroid hormones on Zn2+ metabolism, Zn2+ uptake studies were carried out in intestinal and renal brush-border membrane vesicles (BBMV). Steady-state Zn2+ transport in intestinal and renal cortical BBMV was increased in hyperthyroid (Hyper-T) rats and decreased in the hypothyroid (Hypo-T) rats relative to euthyroid (Eu-T) rats. In both the intestinal and renal BBMV, Hyper-T rats showed a significant increase in maximal velocity compared with Eu-T and Hypo-T rats. Apparent Michaelis constant was unaltered in intestinal and renal BBMV prepared from the three groups. Fluorescence anisotropy of diphenyl hexatriene was decreased significantly in intestinal and renal brush-border membrane (BBM) isolated from Hyper-T rats compared with Hypo-T and Eu-T rats. A significant reduction in the microviscosity and transition temperature for Zn2+ uptake in intestinal and renal BBM from Hyper-T rats is in accordance with the increased fluidity of these BBMs. These findings suggest that the increased rate of Zn2+ transport in response to thyroid hormone status could be associated with either an increase in the number of Zn2+ transporters or an increase in the active transporters due to alteration in the membrane fluidity. Thus the thyroid hormone-mediated change in membrane fluidity might play an important role in modulating Zn2+ transport activity of intestinal and renal BBM.

Unidirectional fluxes of short-chain fatty acids across segments of the large intestine in pig, sheep and pony compared with guinea pig

Unidirectional fluxes of short-chain fatty acids across pig, sheep and pony caecum, proximal and distal colon were studied under short-circuit current conditions in Ussing chambers. Findings are compared with results from guinea pig. Marked species differences are apparent; highest mucosal-to-serosal fluxes of acetate, propionate and butyrate were seen in guinea pig, lower values in pig and smallest fluxes in sheep and pony. Segmental differences between caecum, proximal and distal colon exist mainly in guinea pig and are less developed in pig, sheep and pony. Inhibition of Na+/H+ exchange by amiloride added to the mucosal solution decreased the mucosal-to-serosal fluxes of short-chain fatty acids clearly in guinea pig caecum and proximal colon, and very little in distal colon. This effect was somewhat less pronounced in pig caecum and distal colon, in caecum and distal colon of sheep and caecum of the pony. In pig, sheep and pony proximal colon and pony distal colon no significant inhibition was observed. Inhibition of the K(+)-H+ ATPase by addition of ouabain to the mucosal solution diminished mucosal-to-serosal fluxes of short-chain fatty acids in the guinea pig distal colon extensively. No comparable inhibition was seen in any of the other segments in the animals studied.

Influence of hypothyroidism induced at prepuberty on epididymal lipids and the number and motility of spermatozoa in rats

Fifteen prepubertal male rats (age 30 days) were divided into three groups of five each: Group 1, hypothyroid (Tx)--rats were thyroidectomized at day 30; Group 2, T4 (L-thyroxine) replacement therapy (Tx+T4)--rats were thyroidectomized at day 30 and treated daily i.m. with T4 (6 micrograms/100 g body weight/day) for 30 days from day 31 to day 60 post-thyroidectomy (age 90 days); Group 3, control--rats were sham-operated and treated with vehicle. The rats from all groups were killed on day 61 post-thyroidectomy or post-sham operation (age 91 days). The serum levels of testosterone, T4 and T3 decreased in the Tx group (p < 0.001). In the Tx+T4 group the levels of T4 and T3 were restored to control values, whereas testosterone levels remained lower than in the control group. Hypothyroidism caused various changes in the levels of epididymal phospholipids and neutral lipids. These were restored differentially or were altered further in the Tx+T4 group. The number and forward motility of spermatozoa, recovered from the cauda epididymis, were decreased significantly (p < 0.01) in the Tx group and were not restored in the Tx+T4 group. This study shows that chronic hypothyroidism, induced at prepuberty and continued for 60 days, causes various changes in the lipid composition of the caput and cauda epididymis and lowers the quality and quantity of spermatozoa in the cauda epididymis. T4 therapy for 30 days, especially during the postpubertal period, did not restore the quality or quantity of spermatozoa and caused differential changes in the levels of epididymal lipids, depending upon the region.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological properties of the hyperthyroid rat heart

We have studied the effects of in vivo administration of different T3 doses to thyroidectomized rats on electrophysiological properties, measured in vitro, of papillary muscle fibers. The treatment with increasing T3 doses was associated with a significant reduction of the action potential duration up to a dose as large as 25 micrograms/100 g body weight every second day. The treatment with larger doses of T3 tended to restore the values of the action potential duration present in animals treated with physiological doses (5 micrograms/100 g body weight every second day). Action potential duration is frequency dependent. As the stimulation rate was increased from 1 to 5 Hz, this duration increased in all groups. However the difference between the rat groups remained significant. The cardiac frequency measured in unanaesthetized rats increased as the T3 doses. Furthermore the intrinsic frequency showed a similar increase, indicating a direct effect of T3 on the pacemaker cells in all thyroid states. The mechanism of this action of the thyroid hormone is not, however clear.

Effect of propylthiouracil-induced hypothyroidism on membranes of adult rat brain

The effect of hypothyroidism on the lipid composition of myelin and synaptosomes isolated from adult rat brain was investigated. The animals were made hypothyroid by adding 0.05% propyl-2-thiouracil to their drinking water for four weeks. This pathological state resulted in a significant increase in the relative percentage of choline glycerophospholipids in synaptosomes with a concomitant decrease in ethanolamine glycerophospholipids as compared to controls. In myelin, hypothyroidism significantly influenced only the relative percentage of sulfatides. The effect of the hypothyroid state on mature brain was also reflected in changes in the membrane fatty acid composition. Myelin and synaptosomes showed an increase in arachidonic (20:4) and eicosatrienoic (20:3) acids and an increase in the fatty acid unsaturation index. Furthermore, the 20:4/20:3 and 20:3/18:2 ratios were lower and higher, respectively, in treated animals. The data indicate that hypothyroidism affects the lipid composition of synaptosomes and myelin even though the effects were less pronounced in myelin. The lipid changes observed in hypothyroidism may be of physiological significance, as it is well known that lipid composition modulates various membrane-bound enzymes, transporters and receptors.

Thyroid hormones control lipid composition and membrane fluidity of skeletal muscle sarcolemma

Sarcolemma membrane lipid phase of skeletal muscles of hyperthyroid animals was compared to that of control (euthyroid) ones. Hyperthyroidism caused 15% decrease in cholesterol and 70% increase in the phospholipid content of the membrane. This was accompanied by the alterations in proportions between individual phospholipid classes, and was followed by changes in the composition of phospholipid fatty acids. The calculated fatty acid unsaturation index was higher for membrane lipid phase of hyperthyroid animals than of euthyroid ones. Thyroxine-induced alterations in the lipid composition of sarcolemma caused changes in the membrane fluidity and the activity of calmodulin-stimulated (Ca(2+)-Mg(2+)-ATPase. Measurements of the steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene indicated that the lipid phase transition of membrane vesicles occurred at 25.9 degrees C and at 28.9 degrees C for preparations isolated from hyperthyroid and euthyroid rabbits, respectively. Arrhenius plot break-point temperature for CaM-stimulated (Ca(2+)-Mg(2+)-ATPase activity was lower in membrane preparations isolated from hyperthyroid (26.9 degrees C) than from euthyroid ones (30.0 degrees C). Thus, the increase of the membrane fluidity presumably caused that the enzyme was characterized by the lower activation energy value. This phenomenon may be viewed as a supplementary mechanism for activation of the enzyme by thyroid hormones to previously reported elevation of the amount of (Ca(2+)-Mg(2+)-ATPase protein exerted by hyperthyroidism (Famulski et al. (1988) Eur. J. Biochem., 171, 363-368; Famulski and Wrzosek (1988) in The Ion Pumps-Structure, Function and Regulation (Stein, W.D., ed.), pp. 355-360, Alan R. Liss, New York).

Effect of hyper- and hypothyroidism on phospholipid fatty acid composition and phospholipases activity in sarcolemma of rabbit cardiac muscle

Lipid content and composition of fatty acids esterified to phospholipids of cardiac sarcolemma isolated from hyperthyroid, hypothyroid and control rabbits were analysed. Hyperthyroidism resulted in a significant reduction of the cholesterol to phospholipid molar ratio as compared to control animals, while hypothyroidism exerted the opposite effect. Complex changes in composition of phospholipid fatty acids observed in hyperthyroid state led to an elevation of the fatty acid unsaturation index over the control value. The unsaturation index value was, however, not affected in the hypothyroid state. Thyroxine hormone administration increased phospholipase A1 and decreased phospholipase A2 activity. The opposite effect was observed in thyreodectomized animals. The effect of changes in sarcolemmal bulk phospholipids upon thyroxine administration or deficiency on regulation of activity of membrane-bound enzymes is discussed.