Electron microscopic studies on hepatic alkaline phosphatase in experimentally induced biliary obstruction of the rat

Abstract

The alterations of the alkaline phosphatase (ALP) activity in the rat liver following bile duct ligation were investigated by electron microscopical techniques. Serum ALP activity reached the maximum at 24 hours after ligation and two isozymes of ALP, high molecular and low molecular one, appeared in the serum. Bile canaliculi became dilated at 48 hours after ligation and the microvilli were destructed and diminished in number. ALP activity was observed almost only on the bile canalicular membrane of the liver cells in the control. On the other hand, in the bile duct-ligated rat, the ALP activity on the canalicular membrane was markedly increased initially, then it appeared on the lateral membrane, and finally on the sinusoidal membrane also. It was not stainable on the canalicular membranes which lacked microvilli. The proposed pathway through which hepatic ALP enters the blood stream in bile duct-ligated rats is as follows: ALP, being synthesized in the microsomes of hepatocytes, is initially transferred to the bile canalicular membrane and diffused to lateral membrane through tight junction, reaches to sinusoidal membrane then released into the blood stream.

Altered membrane glycoprotein targeting in cholestatic hepatocytes

“…Electron microscopy shows that in cholestatic hepatocytes, irregular dilation of the bile canaliculi lacking microvilli is accompanied by widening of the pericanalicular ectoplasm, hypertrophy of the smooth endoplasmic reticulum and severe alterations of the tight junctions and cytoskeleton [6,18,19]. Phosphatase activity has not been observed in cholestatic canaliculi [5,20]. In cholestatic hepatocytes, the fact that despite an increase in ALP on the basolateral membrane, a reduction in lectin reactivity on the membrane was still observed, may be ascribable to the loss of membrane (microvilli) rich in sialylated glycoproteins.…”

“…Thus, it can be speculated that in cholestasis, the strong increase in serum ALP is resulting from the direct release of the enzyme from the sinusoidal membrane to the Disse’s space and then to the blood plasma. This would exclude ALP release from the canalicular membrane to lateral membrane through impaired tight junctions, and subsequent release into the bloodstream by sinusoidal membrane [5]. Structural and functional alterations of tight junctions occurring during cholestasis [30] may also be involved in the redistribution of ALP and other membrane (glyco)proteins.…”

“…Alterations in the expression of hepatocytes membrane proteins may indicate disorders of the liver function. Consistent with this idea, the strong increase in plasma ALP characterizing the cholestatic diseases [3,4] is associated with alterations in the distribution of ALP in the lateral and sinusoidal domains of hepatocyte plasma membrane [5]. However, the mechanism underlying the release of ALP into the bloodstream remains poorly understood.…”

“…However, the mechanism underlying the release of ALP into the bloodstream remains poorly understood. It has been hypothesized [5] that the neo‐synthesized ALP is translocated to the canalicular membrane and moves from there to the lateral domain through impaired tight junctions, before reaching the sinusoidal membrane and being released into the bloodstream [5]. Nevertheless, alternative pathways may also exist as suggested by the fact that the cholestatic hepatocyte undergoes ultrastructural alterations that affect the intracytoplasmic vesicular trafficking towards the canalicular membrane [6].…”

Silymarin as a New Hepatoprotective Agent in Experimental Cholestasis: New Possibilities for an Ancient Medication

“…In human hepatopathies, substantial alterations of tight-junctional proteins occur in primary biliary cirrhosis (predominantly in bile ducts) and in primary sclerosing cholangitis (predominantly in hepatocytes) [163]. This may explain the occurrence in these hepatopathies of high plasma levels of solutes confined otherwise to the biliary space, like hepatic enzymes (e.g., alkaline phosphatase) [164] or biliary lipoproteins (forming lipoprotein-X in plasma) [165].…”