The role of catecholamines in calcium homeostasis in fish hepatocytes.

Abstract

The role of Ca$\sp{2+}$ in catecholamine actions on hepatocyte metabolism was studied in three fish species: American eel, brown bullhead and rainbow trout. The cellular Ca content, Ca$\sp{2+}$ fluxes across the cell membrane and cytosolic free-Ca$\sp{2+}$ concentration ((Ca$\sp2\rbrack\sb{\rm i}$) changes in Fura-2-loaded single hepatocytes were measured respectively using atomic absorption spectrophotometer, a $\sp{45}$Ca exchange technique and a computer-controlled microspectrofluorimeter technique. The results indicated the following about Ca$\sp{2+}$ and hepatocyte metabolism. First, fish hepatocytes contain higher Ca content than equivalent mammalian cells (8.25 $\pm$ 1.03 (eel) and 10.49 $\pm$ 1.26 (bullhead) $\mu$moles$\cdot$g$\sp{-1}$ wet wt., respectively). Second, Ca$\sp{2+}$ uptake is a passive or energy-independent process whereas Ca$\sp{2+}$ efflux may be an active or energy-dependent process. Third, Ca$\sp{2+}$ uptake was not significantly stimulated by the catecholamines studied. Fourth, Ca$\sp{2+}$ efflux was significantly stimulated by both epinephrine and phenylephrine in eel hepatocytes and these effects were blocked by the $\alpha$-antagonist phentolamine. The $\alpha$-agonist isoproterenol and $\beta$-antagonist propranolol did not affect basal or hormone-stimulated Ca$\sp{2+}$ efflux. Fifth, basal (Ca$\sp{2+}\rbrack\sb{\rm i}$ was similar in eel and trout hepatocytes, but significantly higher in bullhead cells (184 $\pm$ 23 nM). Sixth, (Ca$\sp{2+}\rbrack\sb{\rm i}$ was significantly increased in eel hepatocytes by either epinephrine or phenylephrine at 10$\sp{-7}$M, but not the $\beta$-agonist isoproterenol (4% increase). Epinephrine and phenylephrine were found to induce repeated $\rm Ca\sb{i}\sp{2+}$ transients (i.e., Ca$\sp{2+}$ oscillations) with variable patterns in individual eel hepatocytes. Trout hepatocytes exhibited little sensitivity to epinephrine with less than 20% of the cells tested showing changes in (Ca$\sp{2+}\rbrack\sb{\rm i}$ in response to catecholamines. Seventh, the initial rise in Ca$\sb{\rm i}\sp{2+}$ induced in eel hepatocytes by epinephrine was independent of external Ca$\sp{2+}$, although external Ca$\sp{2+}$ is required for the long-term maintenance of Ca$\sp{2+}$ oscillations. Bullhead cells depend totally on external Ca$\sp{2+}$ for increasing Ca$\sb{\rm i}\sp{2+}$. (Abstract shortened by UMI.)