Modulation of the hepatic α¹‐adrenoceptor responsiveness by colchicine: dissociation of free cytosolic Ca²⁺‐dependent and independent responses

Abstract

1 The cytoskeletal depolymerizing agent, colchicine, prevents the hepatic ax-adrenoceptor-mediated stimulation of respiration, H+ and Ca2+ release to the effluent perfusate, intracellular alkalosis, and glycogenolysis. Unlike the other parameters, colchicine does not perturb the a,-agonist-induced stimulation of gluconeogenesis or phosphorylase 'a' activation, and enhances the increase in portal pressure response. The lack of effect of colchicine on the hepatic a2-adrenoceptor-mediated effects indicates that its actions are a,-specific.2 Colchicine enhances the acute ax-adrenoceptor-mediated intracellular Ca21 mobilization and prevents the activation of protein kinase C. This differential effect on the two branches of the a1-adrenoceptor signalling pathway is a distinctive feature of the colchicine action.3 The lack of effect of colchicine in altering the ax-adrenoceptor ligand binding affinity suggests that it might interact with some receptor-coupled regulatory element(s). 4 The acuteness of the colchicine effect and the ability of its isomer fl-lumicolchicine to prevent all the ac-adrenoceptor-mediated responses but the increase in vascular resistance, indicate that its action cannot be merely ascribed to its effects in depolymerizing tubulin. 5 Colchicine perturbs the hepatic responses to vasoactive peptides. It enhances the vasopressin-induced rise of cytosolic free Ca2+ in isolated hepatocytes and prevents the sustained decrease of Ca2+ in the effluent perfusate. It also inhibits the stimulation of glycogenolysis, without altering the stimulation of gluconeogenesis.6 It is concluded that there are at least two major a1-adrenoceptor signalling pathways. One is colchicine-sensitive, independent of variations in free cytosolic Ca2+, and protein kinase C-dependent; the other one is colchicine-insensitive, dependent on variations in free cytosolic Ca2", and protein kinase C-independent.

Rapid Flow‐Induced Responses in Endothelial Cells

“…Furthermore, in agreement with our findings on colchicine-treated endothelial cells ( Figure 15B), this drug induced only very moderate effects on intracellular calcium in resting rat brain synaptosomes (72) or cerebellar granule cells (73). However, colchicine has been shown to modulate the calcium permeability of some ion channels in response to chemical stimulation (73)(74)(75). Perhaps colchicine may exert a similar effect on putative calcium channels on the nuclear envelope.…”

3,5,3′-Tri-iodo-l-thyronine acutely regulates a protein kinase C-sensitive, Ca2+-independent, branch of the hepatic α1-adrenoreceptor signalling pathway

“…We have recently reported that activation of hepatic α " -adrenoreceptors is accompanied by activation of the Na + \H + exchanger, resulting in extracellular acidification and intracellular alkalinization [23,24,27]. In agreement with these reports, Figure 8 shows that activation of α " -adrenergic or angiotensin II receptors produced intracellular alkalinization in isolated hepatocytes (Figure 8, left-hand traces).…”

“…Intracellular [Ca# + ] and pH were determined using the intracellular trappable fluorescent Ca# + and pH indicators fura 2 and 2h,7h-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF), respectively, [25,26]. Incubation, loading of the hepatocytes with fura2 acetoxymethyl ester or BCECF-AM and the measurements of agonistmediated changes in cytosolic free Ca# + levels and pH i were performed as described previously [24,27]. To measure Ca# + inflow, liver cells were depleted of internal Ca# + by incubation in nominally Ca# + -free medium in the presence of the endomembranes Ca# + -ATPase inhibitor, thapsigargin [28].…”

Different Subtypes of α1-Adrenoceptor Modulate Different K+ Currents via Different Signaling Pathways in Canine Ventricular Myocytes

“…Intriguingly, a study performed in a vascular smooth muscle cell line (AC01) (18) demonstrated that the ␣ 1D ARs, although representing the minor population compared with the ␣ 1B ARs in this cell, are the main mediators of phosphoinositide/Ca 2ϩ signaling. Moreover, based on their experimental data from isolated hepatocytes, Butta et al (45) concluded that there are at least two major ␣ 1 AR signaling pathways; one is PKC-dependent and independent of variations in free cytosolic Ca 2ϩ , and the other one is dependent on variations in free cytosolic Ca 2ϩ but is PKCindependent. Actually, the ability of ␣ 1 AR to activate CaMK has been previously realized.…”