异种肝细胞移植缓解大鼠急性肝功能衰竭及其排斥反应观察

目的探讨异种肝细胞移植对大鼠急性肝功能衰竭防治效果及其免疫排斥反应。方法将异种豚鼠肝细胞,90%肝除术前1d植入大鼠脾脏内。观察受试大鼠存活时间,及切肝术后24h血生化改变。另外观察植入肝细胞被排斥情况及受体CH50、异种抗体IgG、IgM水平变化。结果(1)中位存活时间,对照组为21h,同种组为56h,异种组为40h。同种组存活时间较对照组延长(P<0·01),异种组亦延长(P<0·05)。(2)异种组血糖和凝血酶原时间改善较明显(P<0·05),同种组谷丙转氨酶、总胆红素、血糖、凝血酶原时间都有明显改善(P<0·05或P<0·01)。(3)受体CH50和异种抗体IgM水平下降与植入异种肝细胞排斥过程同步。结论异种肝细胞移植对大鼠急性肝功能衰竭有防治作用,补体和异种抗体IgM与排斥反应关系密切。     

静脉注射血管内皮细胞对异种心脏移植的影响

目的　探讨诱导非协调性异种移植免疫耐受的新方法。方法　移植前 1 4d外周静脉注射抗原量 1 0× 1 0 6 个豚鼠血管内皮细胞或联合腹腔注射环磷酰胺 (2 0mg·kg- 1 ·d- 1 × 1 4d)诱导豚鼠 大鼠异种心脏移植免疫耐受 ,观察供心存活时间。结果　外周静脉注射豚鼠血管内皮细胞联合使用环磷酰胺能够延长豚鼠 大鼠异种心脏移植存活时间 ,其平均存活时间为 (67.5± 6 .4)min ,与其余各组相比 ,差异有显著性 (P <0 .0 5)。供心标本免疫组织化学检测显示豚鼠血管内皮细胞外周静脉注射联合使用环磷酰胺组抗体IgM及补体C3沉积均较其余组少 ,定量测定分别为40 .5± 3 .9,46 .8± 5 .2 ,与其余各组相比 ,差异有显著性 (P <0 .0 5)。结论　移植术前外周静脉注射血管内皮细胞联合使用免疫抑制剂能够延长非协调性异种心脏移植存活时间 ,血管内皮细胞可以作为新的耐受原诱导免疫耐受     

受体血清“封闭”供肝预防异种肝移植超急性排斥反应

目的 探讨受体血清“封闭”供肝对异种肝移植超急性排斥反应(hyperacute rejection,HAR)的预防作用.方法 取豚鼠和SD大鼠各20只,分别作为供体和受体,供受体随机配对;移植前采集受体大鼠近交系其他个体血清,45℃水浴灭活补体备用;实验组(n=10)术前用0.1％受体血清(recipient serum,RS)的Ringer液“封闭”供肝,对照组(n=10)仅用Ringer液灌洗供肝;采用改良“双套管法”行豚鼠、SD大鼠原位肝移植,观察供肝植入后形态学改变、受体存活时间、术后1h存活率,HE染色法检测移植肝微血栓、出血和肝细胞水肿等病理损害积分;检测血清丙氨酸转氨酶(ALT)评价肝功能.结果 两组大鼠供体异种肝移植时间和无肝期比较,差异无统计学意义(P＞0.05);对照组受体大鼠供肝充盈缓慢,灌注不均,实验组供肝充盈迅速,灌注较均匀;实验组受体大鼠术后存活时间和术后1h存活率较对照组均明显增加(P＜0.01),移植肝微血栓、间质出血(积分)较对照组明显减轻(P＜0.01),肝细胞水肿无明显差异(P＞0.05);血清丙氨酸转氨酶(ALT)较对照组明显下降(P＜0.05).结论 受体血清“封闭”供肝对异种肝移植HAR具有一定的抑制作用,是预防器官移植HAR的潜在方法之一.     

应用环孢素A治疗异种肝细胞移植的排斥反应

目的　探讨应用环孢素A(CsA)治疗异种肝细胞移植排斥反应的疗效及其排斥机制。方法　将豚鼠肝细胞植入D-氨基半乳糖诱导的肝衰大鼠脾内,分CsA治疗组和单纯移植组。观察2周生存率及脾病理组织学检查;酶联免疫双抗体夹心法(Sandwich-ELISA)测定大鼠血清白细胞介素(IL)-2的浓度。结果　2周生存率比较CsA治疗组（78.6%）与单纯移植组（80.0%）相比差异无显著性(P＞0.05)。应用CsA可减缓炎症细胞浸润。血清IL-2浓度检测正常大鼠为(28.7±7.0)ng/L,移植后各组血清IL-2浓度变化不大,直至移植后第7天,CsA治疗组为(28.5±6.0)ng/L、单纯移植组为(31.5±8.0)ng/L。3组间差异均无显著性(P＞0.05)。结论　在异种肝细胞脾内移植中,细胞免疫发生的较迟和/或较弱。单独应用CsA治疗异种肝细胞脾内移植引发的排斥反应其疗效不佳。     

非协调性异种肝移植动物模型的建立:动脉化模型与静脉化模型的比较

目的 建立豚鼠至大鼠非协调性异种肝移植动物模型,并对动脉化模型和静脉化模型进行比较.方法 共进行异种肝移植40次,其中20对为动脉化组,20对为静脉化组.比较两组存活时间、受体血中肝脏酶的变化,移植肝组织学改变和荧光抗体染色.结果 动脉化组存活时间为(92.95±28.52) min,静脉化组为(135.10±46.12) min.两组移植肝组织学变化基本相似,肝细胞水样变形、血管和血窦淤血;荧光染色见IgM和IgG沉积于血管内皮细胞和肝血窦.结论 在没有克服超急性排斥反应之前,豚鼠至大鼠异种肝移植静脉化模型比动脉化模型更简单实用.     

延长大鼠异种心脏移植后存活时间的实验研究

目的　研究如何延长大鼠异种心脏移植后的存活时间。方法　实验分为A、B、C、D四组。A组 :移植术前 12、8、4、0d及 10、6、2、0d分别将脾细胞 1× 10 8个 ,抗血清 0 .2ml静脉注入受体大鼠 ;B组 :在A组的基础上 ,加用中华眼镜蛇毒 (CCV) 0 .2mg·kg-1·d-1,术前 3d至术日腹腔注射。C组 :在B组的基础上 ,加用环孢素A(CsA) 10mg·kg-1·d-1、环磷酰胺 (Cy) 2 0mg·kg-1·d-1,术前 12d开始至术日腹腔注射。D组 :在C组的基础上 ,加用抗巨噬细胞和抗自然杀伤细胞单克隆抗体 2 5 0 μg·kg-1·d-1,术前 12d开始至术日腹腔注射。结果　A、B、C、D四组移植心脏分别存活 (0 .32± 0 .12 )h ,(2 5 .6± 9.6 )h、(48.6± 10 .4)h和 (72 .4± 2 1.7)h ;术日各组IgG均下降 ,尤以C、D组下降明显 ,与A、B组比较 ,差异有显著性 (P <0 .0 5 )。结论　术前静脉注射供体脾细胞及抗血清 ,尤其与CCV、CsA、Cy合用 ,能显著抑制IgG的产生 ,延长移植心脏的存活时间。     

异种肝细胞移植治疗大鼠药物性肝衰的疗效观察

目的　观察异种肝细胞移植治疗大鼠药物性肝衰的疗效。方法　杂种豚鼠为供体 ,胶原酶消化法制备肝细胞。SD大鼠为受体 ,氨基半乳糖 (D GI)腹腔内 1次注射制作肝衰模型。 4 8h后将豚鼠肝细胞(1.5× 10 7个 )移植于大鼠脾内。同种移植及生理盐水为对照。移植后观察受体 2周存活率 ,在移植不同时间作移植物病理及组织化学检查。结果　受体 2周存活率 :异种移植组为 71% ,同种组为69 % (P >0 .0 5 ) ,生理盐水对照组为 2 5 % (P>0 .0 1)。豚鼠肝细胞移植后 12～ 2 4h其结构和功能基本保存完好。结论　与同种移植一样 ,异种肝细胞移植能逆转大鼠药物性肝衰。     

非协调性异种肝移植中移植肝超急性排斥反应的病理形态学表现

目的:了解豚鼠至大鼠异种肝移植动物模型中移植肝病理学表现。方法:利用血管套技术和显微外科技术进行了20例豚鼠至大鼠肝移植。观察了受体存活时间和移植肝HE染色情况、电子显微镜下表现和荧光染色表现。用TUNEL法检测了移植肝细胞凋亡情况。结果:受体存活时间平均为(135.10±46.12)min,组织学表现为肝小叶结构存在,但肝细胞发生弥漫性水样变性,中央静脉和小叶间静脉淤血;电子显微镜下见肝细胞内糖原颗粒消失,细胞器水肿,近血窦处肝细胞膜破坏;IgM和IgG染色主要位于肝脏血管内皮细胞表面和肝血窦内。细胞凋亡指数为(0.80±0.31)％。结论:首次建立了豚鼠至大鼠异种肝移植模型,观察了超急性排斥反应。     

FK506对非协调性异种肝移植供肝再灌注障碍的作用

目的　探讨FK5 0 6对非协调性异种肝移植再灌注障碍的防治作用。方法　对豚鼠至SD大鼠非协调性异种肝移植进行FK5 0 6治疗 ,观察FK5 0 6治疗组异种肝移植后肝脏门脉血流变化和存活时间及病理变化。结果　发现FK5 0 6可以减缓异种肝移植后肝脏门静脉血流的降低 ,未使用FK5 0 6组受体存活时间为 15 7± 7.1(min) ,而FK5 0 6治疗组为 2 15 .9±16 .0 (min) ,两组 p <0 .0 1。 结论　FK5 0 6可减轻非协调性异种肝移植移植后肝脏的灌注障碍 ,延长存活时间。     

异种肝细胞移植防治大鼠急性肝功能衰竭

目的: 探讨异种肝细胞移植对大鼠急性肝功能衰竭的治疗效果. 方法: 切除大鼠90%肝脏,建立急性肝功能衰竭模型.分离异种豚鼠和同种异体Sprague-Dawley鼠肝细胞,切肝术前1天植入受体脾脏内.观察受试大鼠存活时间、切肝术后24小时血生化改变和脾脏切片表现. 结果: ①中位存活时间,对照组为21小时,同种组为56小时,异种组为40小时.同种组存活时间较对照组长(P<0.01),异种组亦延长(P<0.05).②移植组部分血生化指标有所改善.异种组中仅血糖和凝血酶原时间改善较明显(P<0.05),同种组中谷丙转氨酶、总胆红素、血糖和凝血酶原时间都有明显改善(P<0.05或P<0.01).③肝细胞植入后48小时,异种组脾脏内仅存少量萎缩的肝细胞,同种组脾脏内仍散在健存肝细胞. 结论: 异种肝细胞移植对大鼠急性肝功能衰竭有防治作用;要提高移植效果,首先需克服早期的免疫排斥.     

Vasopressor hormone response following mesenteric traction during major abdominal surgery

Background : We investigated the vasopressor hormone response following mesenteric traction (MT) with hypotension due to prostacyclin (PGI₂) release in patients undergoing abdominal surgery with a combined general and epidural anesthesia. Methods : In a prospective, randomized, placebo-controlled study we administered 400 mg ibuprofen (i.v.) in 42 patients scheduled for abdominal surgery. General anesthesia was combined with epidural anesthesia (T4-L1). Before as well as 5, 15, 30, 45, and 90 min after MT we recorded plasma osmolality, hemodynamics and measured 6-keto-PGFlα (stabile metabolite of PGI₂), TXB₂ (stabile metabolite of thromboxane A₂) active renin, and arginine vasopressin (AVP) plasma concentrations by radioimmunoassay. Catecholamine levels were assessed by high-pressure liquid chromatography (HPLC) with electrochemical detection. Results : Following MT, arterial hypotension occurred along with a substantial PGI₂ release. This was completely abolished by ibuprofen administration. Although plasma levels of 6-keto-PGF_1α (1133 (708) vs. 60 (3) ng/L, median (median absolute deviation), P=0.0001, placebo vs. ibuprofen) remained significantly elevated, blood pressure was restored within 30 min after MT in the placebo group. At the same point in time plasma concentrations of TXB² (164 (87) vs. 58 (1) ng/L, P=0.0001), epinephrine (46 (33) vs. 14 (6) ng/L, P=0.001), AVP (41 ± (18) vs. 12 (7) ng/L, P=0.0004), and active renin (27 (12) vs. 12 (4) ng/L, P = 0.001) were significantly higher in placebo-treated patients. Conclusion : Under combined general and epidural anesthesia arterial hypotension following MT due to endogenous PGI₂ release is associated with enhanced release of AVP, active renin, epinephrine and thromboxane A₂, presumably contributing to hemodynamic stability within 30 min after MT.     

A Teaspoon Pump for Pumping Blood with High Hydraulic Efficiency and Low Hemolysis Potential

Abstract: Virtually all blood pumps contain some kind of rubbing, sliding, closely moving machinery surfaces that are exposed to the blood being pumped. These valves, internal bearings, magnetic bearing position sensors, and shaft seals cause most of the problems with blood pumps. The original teaspoon pump design prevented the rubbing, sliding machinery surfaces from contacting the blood. However, the hydraulic efficiency was low because the blood was able to "slip around" the rotating impeller so that the blood itself never rotated fast enough to develop adequate pressure. An improved teaspoon blood pump has been designed and tested and has shown acceptable hydraulic performance and low hemolysis potential. The new pump uses a nonrotating "swinging" hose as the pump impeller. The fluid enters the pump through the center of the swinging hose; therefore, there can be no fluid slip between the revolving blood and the revolving impeller. The new pump uses an impeller that is comparable to a flexible garden hose. If the free end of the hose were swung around in a circle like half of a jump rope, the fluid inside the hose would rotate and develop pressure even though the hose impeller itself did not "rotate"; therefore, no rotating shaft seal or internal bearings are required.     

Microspheres Based Detoxification System: A New Method in Convective Blood Purification

Abstract: A variety of protein-bound or hydrophobic substances, accumulating as a result of pathologic conditions such as exogenous or endogenous intoxications, are removed poorly by conventional detoxification methods because of low accessibility (hemodialysis), insufficient adsorption capabilities (hemosorption), low efficiency (peritoneal dialysis), or economic limitations (high-volume plasmapheresis). Combining advantages of existing methods with microspheric technology, a module-based system was designed. Major operating parameters of the latter can be modified to allow for adjustment to individual clinical situations. An extracorporeal blood circuit including a plasmafilter is combined with a secondary high-velocity plasma circuit driven by a centrifugal pump. Different microspheric adsorbers can be combined in one circuit or applied in sequence. Thus, a prolonged treatment can be tailored using specially designed selective adsorber materials. Comparing this system with existing methods (high-flux hemodialysis, molecular adsorbent recycling system), results from our in vitro studies and animal experiments demonstrate the superior efficiency of substance removal.     

Tissue perfusion in relation to cardiac output during continuous positive-pressure ventilation and administration of propranolol or verapamil

Background : Our objective was to determine whether administration of propranolol or verapamil modifies the hemodynamic adaptation to continuous positive-pressure ventilation (CPPV), in particular the regional distribution of cardiac output (CO).
Methods : General hemodynamics and regional blood flows assessed by microsphere technique (15 (μm) were recorded in 16 anesthetized pigs during spontaneous breathing (SB) and CPPV with 8 cm H₂O end-expiratory pressure (CPPV8) before and after intravenous administration of propranolol (0.3 mg · kg⁻¹ followed by 0.15 mg · kg⁻¹ · h⁻¹, n=8) or verapamil (0.1 mg · kg⁻¹ followed by 0.3 mg · kg⁻¹ · h⁻¹, n=8).
Results : CPPV8 depressed CO by 25% without shifts in its relative distribution with the exception of a noteworthy increase in adrenal perfusion. Propranolol increased arterial blood pressure, and due to a fall in heart rate, CO dropped by 25%. The kidneys and, to a lesser extent, the splanchic region and central nervous system received increased fractions of the remaining CO at the expense of skeletal muscle flow. Similar patterns were seen during SB and CPPV8 such that the combination of propranolol and CPPV8 depressed CO by 50%. The circulatory effects of verapamil were less evident but myocardial perfusion tended to increase.
Conclusions : The combination of propranolol or verapamil with CPPV does not result in any specific hemodynamic interaction in anesthetized pigs, except that the combined effect of propranolol and CPPV may severely reduce CO.     

Bariatric Surgery in Some "Central and East-European (former Eastern bloc)"Countries - Current Status and Prediction for the Next Millennium

Background: Obesity is increasing globallly, including in the formerly "Eastern Bloc" countries. Methods: A survey was made of obesity and bariatric surgery. Results: In the 8 East and Central European countries studied, with total population 300 million, roughly 43% of the population was overweight (BMI 25-30), 23% obese (BMI > 30), with about 15 million people morbidly obese (BMI > 40). From 0-10 morbidly obese individuals/100,000/year undergo bariatric surgery. Conclusion: Most countries were found to provide inadequate treatment for obesity.The majority of the morbidly obese are not treated effectively. However, health-care awareness of obesity and bariatric surgeons are slowly increasing.     

Volatile anaesthetics reduce adhesion of blood platelets under low-flow conditions in the coronary system of isolated guinea pig hearts

Background : Inhibitory effects of volatile anaesthetics on platelet aggregation have been demonstrated in several studies. However, the influence of volatile anaesthetics on intracoronary platelet adhesion has not been elucidated so far.
Methods : Isolated hearts of guinea pigs were perfused with buffer in the absence or presence of volatile anaesthetics (0.5 and 1 MAC) at constant coronary flow rates of 5 ml/min for 25 min, then 1 ml/min for 30 min and again 5 ml/min for 10 min. Before, during and after low-flow perfusion, a bolus of human platelets was applied into the coronary system. To simulate thrombogenic conditions, 0.3 U/ml human thrombin was infused during low-flow perfusion and reperfusion. The number of platelets sequestered to the endothelium was calculated from the difference between coronary in- and output of platelets. The myocardial production of lactate and consumption of pyruvate and coronary perfusion pressure were also determined.
Results : At a flow rate of 5 ml/min only about 3% of the applied platelets did not emerge from the coronary system, in any group. In contrast, 13.1±1.2% (mean±SEM) of infused platelets became adherent in low-flow perfusion in the control group without anaesthetic. The adherence was reduced with each 1 MAC isoflurane (to 6.2±1.2%), sevoflurane (to 4.4±0.9%) or halothane (to 3.2±1.5%) (each P <0.05 vs. control). Volatile anaesthetic, 0.5 MAC, did not inhibit platelet adhesion to a statistically significant extent in any case. Perfusion pressure and metabolic parameters were not statistically different between the control and the hearts exposed to anaesthetics.
Conclusion : Volatile anaesthetics in a concentration of 1 MAC can reduce the adhesion of platelets in the coronary system under reduced flow conditions. This action does not arise from vasodilation or inhibition of ischaemic stress.     

Synergistic interaction between the effects of propofol and midazolam with fentanyl on phrenic nerve activity in rabbits

Background: It has been shown that the depressive effects of both propofol and midazolam on consciousness are synergistic with opioids, but the nature of their interactions on other physiological systems, e. g. respiration, has not been fully investigated. The present study examined the effect of propofol and midazolam alone and in combination with fentanyl on phrenic nerve activity (PNA) and whether such interactions are additive or synergistic. Methods: PNA was recorded in 27 anaesthetised and artificially ventilated rabbits. In three groups, propofol, fentanyl and midazolam were administered intravenously in incremental doses to construct dose-response curves for the depressant effects of each one on PNA. In another two groups, the effect of pretreatment with either fentanyl 1 μg · kg^?1 i. v. or midazolam 0.05 mg · kg^?1 i. v. on the effects of propofol and fentanyl respectively on PNA were studied. Results: Propofol and fentanyl caused a dose-dependent depression of PNA with complete abolition at the highest total doses of 16 mg · kg^?1 i. v. and 32 μg · kg^?1 i. v., respectively. In contrast, midazolam in incremental doses to a total of 0.8 mg · kg^?1 reduced mean PNA by 63%, but approximately 12% of PNA remained at a total dose as high as 6.4 mg · kg^?1. The mean ED₅₀s, calculated from dose-response curves, were 5.4 mg · kg^?1, 3.9 μg · kg^?1 and 0.4 mg · kg^?1 for propofol, fentanyl and midazolam, respectively. Initial doses of either fentanyl 1 μg · kg^?1 i. v. or midazolam 0.05 mg · kg^?1 i. v. acted synergistically with subsequent doses of either propofol or fentanyl to abolish PNA at total doses of 8 mg · kg^?1 and 8 μg · kg^?1, respectively. Conclusion: Fentanyl has a synergistic interaction with both propofol and midazolam on PNA and hence potentially on respiration.     

Influence of dopexamine hydrochloride on haemodynamics and regulators of circulation in patients undergoing major abdominal surgery

Background: Catecholaminergic support is often used to improve haemodynamics in patients undergoing major abdominal surgery. Dopexamine is a synthetic vasoactive catecholamine with beneficial microcirculatory properties. Methods: The influence of perioperative administration of dopexamine on cardiorespiratory data and important regulators of macro- and microcirculation were studied in 30 patients undergoing Whipple pancreaticduodenectomy. The patients received randomized and blinded either 2 μg · kg^?1 · min^?1 of dopexamine (n=15) or placebo (n=15, control group). The infusion was started after induction of anaesthesia and continued until the morning of the first postoperative day. Endothelin-1 (ET-1), vasopressin, atrial natriuretic peptide (ANP), and catecholamine plasma levels were measured from arterial blood samples. Measurements were carried out after induction of anaesthesia, 2 h after onset of surgery, at the end of surgery, 2 h after surgery, and on the morning of the first postoperative day. Results: Cardiac index (CI) increased significantly in the dopexamine group (from 2.61±0.41 to 4.57±0.78 1 · min^?1 · m^?2) and remained elevated until the morning of the first postoperative day. Oxygen delivery index (DO₂I) and oxygen consumption index (VO₂I) were also significantly increased in the dopexamine group (DO₂I: from 416±91 to 717±110 ml/m² · m²; VO₂I: from 98±25 to 157±22 ml/m² · m²), being significantly higher than in the control group. pHi remained stable only in the dopexamine patients, indicating adequate splanchnic perfusion. Vasopressive regulators of circulation increased significantly only in the untreated control patients (vasopressin: from 4.37±1.1 to 35.9±12.1 pg/ml; ET-1: from 2.88±0.91 to 6.91±1.20 pg/ml). Conclusion: Patients undergoing major abdominal surgery may profit from prophylactic perioperative administration of dopexamine hydrochloride in the form of improved haemodynamics and oxygenation as well as beneficial influence on important regulators of organ blood flow.     

Systemic analgesia adapted to the children's condition

A concept of balanced analgesia using nonsteroidal anti-inflammatory drugs (NSAIDs), paracetamol (acetaminophen), opioids, and corticosteroids can also be used in patients with pre-existing illnesses. NSAIDs are the most effective treatment for acute pain of moderate intensity in children; however, these drugs should be avoided in patients at increased risk for serious side effects, e.g. patients with renal impairment, bleeding tendency, or extreme prematurity. NSAIDs can be given with minimal risks to the younger child with mild to moderate asthma, and, in these patients, the use of steroids can be encouraged; in addition to their antiemetic and analgesic action, a beneficial effect on asthma symptoms can be expected. In the non-intubated child with cerebral trauma, exaggerated sedation caused by opioids and increased bleeding tendency caused by NSAIDs must be avoided. In neonates and small infants, the oral administration of sucrose or glucose is helpful to minimize pain reaction during short uncomfortable interventions.     

A comparison of the inhibitory effects of four volatile anaesthetics on the metabolism of chlorzoxazone, a substrate for CYP2E1, in rabbits

Background: Halothane inhibits in vitro and in vivo activity of cytochrome P-450 (CYP) 2E1. There are several fluorinated volatile anaesthetics besides halothane, and most of them are defluorinated by CYP2E1. It is unclear whether other fluorinated anaesthetics inhibit the in vivo activity of CYP2E1.
Methods: We compared the inhibitory effects of therapeutic concentrations of four inhalational anaesthetics, halothane, enflurane, isoflurane, and sevoflurane, on chlorzoxazone metabolism in rabbits receiving artificial ventilation.
Results: All four inhalational anaesthetics decreased arterial blood pressure and increased plasma chlorzoxazone concentration. However, no significant differences in the plasma chlorzoxazone concentration were found between the four anaesthetics. The estimated chlorzoxazone clearance increased after beginning inhalation with all four agents, but no significant difference in clearance was noted between agents.
Conclusions: At therapeutic concentrations, the in vivo inhibitory effect on chlorzoxazone metabolism was similar for all four inhalational anaesthetics examined, even though their chemical characteristics and extent of hepatic metabolism differ considerably.