Michael D. Apstein

Pathogenesis of cholesterol gallstones: a parsimonious hypothesis.

Three decades of fundamental research on cholesterol gallstones have borne witness to a continuous growth of basic and clinical information on all aspects of the disease [1–5]. Unfortunately, with respect to pathogenesis our understanding is imperfect and often unintentionally confusing and conflicting [1,2]. Furthermore, according to the current paradigm [6] the simultaneous occurrence of several independent defects to explain all pathophysiological elements in the disease appears excessive [7]. In this article we propose a unified hypothesis for cholesterol gallstone pathogenesis that is realistic on the basis of recent biophysical [8] and genetic [9] insights. Our hope is that eventually Occam’s razor or the law of parsimony (1836), can be put to use in understanding this common disease. The current working hypothesis on pathogenesis [6– 8,10] involves a tetralogy of simultaneous defects. They are (1) cholesterol hypersecretion with unphysiological supersaturation of bile, (2) gallbladder hypomotility (‘biliary stasis’), (3) accelerated cholesterol crystallization in bile and (4) mucus hypersecretion with gel formation in the gallbladder [11]. The severity of each of these defects may vary markedly [12] and they have not been considered to be interrelated to date. As first demonstrated 20 years ago [13], bile becomes supersaturated with cholesterol because of hypersecretion of hepatic cholesterol into bile [5]. Cholesterol monohydrate crystal formation occurs rapidly in the lithogenic gallbladder because of accelerated nucleation and phase transitions [14]. A poorly contractile and compliant gallbladder that neither fills nor empties adequately in the lithogenic state [15] allows cholesterol crystals to become entrapped in a mucin gel (‘biliary sludge’) [11]. After months to years, crystals agglomerate to form macroscopic gallstones [8]. It is inconceivable that such a large number of hepatobiliary abnormalities [8] could have independent causes simultaneously [7]. Furthermore, in the modern era of cholesterol gallstone research, the earliest metabolic defect discovered was a small but rapidly circulating bile salt pool accompanied by increased primary bile salt turnover rates [16]. As primary bile salts are synthesized in the liver from cholesterol [5], the dilemma for most investigators was to understand how a liver as well as an intestinal defect could be related to explain the enterohepatic circulatory abnormalities in gallstone disease. In this review, we attempt to marshall the available evidence to demonstrate that hepatic hypersecretion of cholesterol may be the major trigger for the entire panoply of pathophysiologic abnormalities responsible for cholesterol gallstone disease [7,8]. Accumulating evidence with respect to gallbladder muscle injury [17–19], qualitative and quantitative alterations in specific biliary proteins [2,11,20–22] and possibly pronucleating critical nuclei of colloidal dimensions [23,24] suggest that molecular cholesterol may in fact touch off the other abnormalities in this disease [7,8]. Furthermore, the combined influences of these secondary defects will be shown to explain the perturbations of the enterohepatic circulation in lithogenesis and which may amplify many of the primary and secondary defects depending on their severity (Fig. 1). The high prevalence rates of cholesterol gallstones in Pima Indians suggested that an inherited defect in cholesterol homeostasis was responsible for the disease [25]. Only recently has genetic analysis of the cholesterol gallstone-susceptible inbred laboratory mouse [9] revealed that at least two Lith genes affect cholesterol gallstone formation. The protein encoded by Lith 1, the European Journal of Clinical Investigation (1996)26, 343–352

Effect of Organic Anions on Biliary Lipids in the Rat

Bile salts enhance the biliary secretion of phospholipid and cholesterol. Other amphipilic molecules, organic anions, are secreted into bile as well. We studied the effects of bilirubin and iodipamide, two chemically dissimilar organic anions, on biliary lipid secretion in the rat. We infused bile salt pool-depleted rats with a stepwise infusion of taurocholate and a constant infusion of organic anion. Both organic anions markedly inhibited the biliary secretion of phospholipid and cholesterol without affecting bile salt secretion. This inhibition, at least with iodipamide, was dose-dependent and fully reversible. Using tritiated water as a precursor, we measured hepatic and biliary cholesterol synthesis in the presence or absence of an iodipamide infusion to see if decreased lipid synthesis could explain decreased secretion. Despite the marked reduction in biliary cholesterol secretion, the specific activity of biliary cholesterol was unchanged during an iodipamide infusion. We suggest that organic anions interfere with the assembly of the biliary mixed micelle resulting in micelles that are deficient in phospholipid and cholesterol.

Ampicillin inhibits biliary cholesterol secretion

There are compounds, other than bile salts, which influence biliary lipid composition. For example, the organic anions, bilirubin, and BSP, are secreted into bile and depress biliary phospholipid and cholesterol secretion. Ampicillin is also secreted into bile, but its effects on biliary lipid secretion are unknown. Therefore we measured biliary lipid secretion before, during, and after an ampicillin infusion. We infused bile salt pool depleted rats with a constant infusion of taurocholate. After biliary lipid secretion reached a steady state, the ampicillin solution was infused in a stepwise fashion. Within minutes after starting the ampicillin, bile flow and biliary ampicillin secretion increased. Simultaneously, biliary phospholipid and cholesterol, but not bile salt, secretion decreased significantly. The calculated lithogenic index of bile decreased significantly as well. Despite doubling the ampicillin infusion, neither biliary ampicillin, phospholipid, nor cholesterol secretion changed. After discontinuation of the ampicillin, bile flow and biliary ampicillin secretion decreased, while phospholipid and cholesterol secretion returned to normal. Ampicillin dramatically influences biliary lipid composition directly. It inhibits the biliary secretion of phospholipid and cholesterol, but not bile salt, and consequently reduces the molar percentage of cholesterol in bile.

Serum lipids during the first year following acute spinal cord injury

Patients with chronic spinal cord injury (SCI) have low levels of high-density lipoprotein (HDL) cholesterol. The cause of this abnormality and its relation to SCI are unknown. We studied 100 consecutively admitted males with acute SCI prospectively for 1 year to determine changes in serum lipids following acute SCI and the relation of these changes to the level of injury and physical activity. Low-density lipoprotein (LDL), HDL, and total cholesterol and triglycerides were all decreased following acute SCI and gradually increased toward normal by 1 year. Serum HDL, LDL, and total cholesterol levels correlated with the level of SCI: levels were lower in quadriplegic compared with paraplegic patients. Serum HDL showed the greatest change following SCI, increasing by 26% in quadriplegic and 18% in paraplegic patients. These changes in serum lipid levels could not be explained by changes in alcohol consumption or body weight. We estimated that only 44% of the increase in HDL following SCI was associated with an increase in physical activity. We postulate that interruption of the autonomic nervous system influences lipid metabolism and serum lipid levels.