Effect of the intracellular Ca+2 antagonist TMB-8 on serum-stimulated Na+ influx in human fibroblasts

The intracellular calcium antagonist TMB-8 [8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate] inhibits mitochondrial ATP production in rat thymocytes

TMB-8 inhibited respiration of rat thymocytes and rat liver mitochondria, probably by inhibition of NADH dehydrogenase. TMB-8 markedly decreased both the cellular ATP concentration and the mitochondrial membrane potential in situ in thymocytes. These effects occurred at, or well below, the concentrations used in other systems to investigate the role of intracellular calcium pools in signalling events. We conclude that caution should be exercised in the interpretation of the effects of TMB-8.

Platelet-derived growth factor stimulates Na+ influx in vascular smooth muscle cells

Platelet-derived growth factor (PDGF) is known to be a potent mitogenic agent for vascular smooth muscle cells. The effect of PDGF on amiloride-sensitive Na+ influx was investigated in A-10 vascular smooth muscle cells. At a dose which maximally stimulates DNA synthesis, PDGF stimulates net Na+ influx in vascular smooth muscle cells. The PDGF-stimulated Na+ influx is linear over a 5-min time course. PDGF stimulates Na+ influx in a concentration-dependent manner, with a concentration of 0.5 U/ml causing half-maximal inhibition. The effects of PDGF on Na+ influx can be mimicked by 10% fetal bovine serum or by the divalent cation ionophore A23187 (5 microM). Net Na+ influx in response to each of these agents was inhibited by amiloride and by benzamil. PDGF-stimulated net Na+ influx was blocked by the intracellular Ca2+ antagonist 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate as well as by the calmodulin antagonists trifluoperazine, chlorpromazine, and imipramine. These data suggest that PDGF may stimulate an amiloride-sensitive Na+ influx pathway via an elevation in intracellular Ca2+ and formation of a Ca-calmodulin complex. This Na+ influx pathway may be a trigger for PDGF-stimulated DNA synthesis.

Modulation of chloroplast movement in the green alga Mougeotia by the Ca2+ ionophore A23187 and by calmodulin antagonists

The Ca2+ ionophore A23187 can induce chloroplast rotation within a single nonirradiated Mougeotia cell. The induced turning was dependent on the position of ionophore application and Ca2+ in the external medium. The role of calmodulin in mediating light-induced chloroplast rotation in the alga Mougeotia was investigated by using the paired calmodulin-antagonist drugs W5-W7 and W12-W13. In each pair, the antagonist with the greater affinity for calmodulin had the greater inhibitor effect on the phytochrome-controlled light response. These results support the hypothesis that calcium functions as a chemical messenger to couple the stimulus of phytochrome photoactivation with physiological responses in plants.

Activation of amiloride-sensitive sodium transport in C6 glioma cells

We have characterized, in C6 cells, an amiloride-sensitive Na+ entry pathway that can exchange for H+. In this report we demonstrate that this cation-exchange system can be induced within 24-36 h by either serum removal or by dibutyryl cyclic AMP; however, these modes of induction are not additive and are manifest only after activation by serum. In these glioma cells we found that activation by serum can be mimicked in part by specific serum factors, i.e., epidermal growth factor and bradykinin. We attempted to characterize this activation process further using several cell biologic probes. We had previously shown that that activation process involves a calcium-dependent step with full activation obtained in the presence of the calcium ionophore A23187. The activation by serum was inhibited by preincubation with colchicine but not with dihydrocytochalasin B, suggesting a cytoskeletal involvement in the activation process. Activation by epidermal growth factor and bradykinin was found to be unaffected by colchicine, suggesting that other factors must be present in serum that confer sensitivity to colchicine. Incubation of the cells with phorbol myristoyl acetate results in the activation of amiloride-sensitive transport, suggesting that stimulation of protein kinase C may be integral to the activation process. Unlike the effects of serum, activation by phorbol myristoyl acetate is not inhibited by colchicine, indicating that this drug works in a way that bypasses the cytoskeletal-dependent step. Since diacylglycerol is the presumed endogenous activator of protein kinase C, we studied the effects of dioleylglycerol. This intermediate of phospholipid turnover was found to increase specifically the amiloride-sensitive sodium pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of the serum effect on Na+ influx in cultured human fibroblasts

Stimulation of an amiloride-sensitive Na+ influx pathway, which mediates Na+/H+ exchange, has been postulated to be an important step in the initiation of DNA synthesis in quiescent human fibroblasts. If the elevation of intracellular Na+ or the alkalinization of intracellular pH resulting from the activation of this system is a trigger for subsequent mitogenic events, then its inactivation may also be important to cellular functions. We investigated the duration of the activation of Na+ influx by serum in human foreskin fibroblasts (HSWP). It was found that activation of Na+ influx by 10% serum was transient, declining with a t 1/2 = 15 min. Similarly, the Na+ content of the cells rose rapidly following serum addition and decreased with a t 1/2 = 15 min. In addition, both the lys-bradykinin- and the vasopressin-stimulated Na+ influx and Na+ content declined with a t 1/2 of approximately 15 min. Similar results were obtained using both Tris-buffered and Hepes-buffered, amino-acid-free EMEM. Finally, the above experiments were repeated under conditions normally used to assess the mitogenic response of cells. It was found that in cells arrested in G0 by serum deprivation in CO2-buffered EMEM, the serum activated Na+ flux was also transient with a t 1/2 of approximately 20 min. The desensitization of cells to serum could be readily (t 1/2 = 20') reversed by a subsequent incubation of cells in serum-free medium. Stimulation of Na+ influx by both the divalent cation ionophore A23187 and the phospholipase activator melittin in also desensitized rapidly, suggesting the process is independent of receptor downregulation. The desensitization during serum preincubation occurred in both low Na+ and low pH medium suggesting that the process is not due to negative feedback on the transport system via a rise in cellular Na+ concentration or a rise in intracellular pH. Although the mechanism of desensitization is at present not known, it is likely to be a physiologically important event.

Serum, bradykinin and vasopressin stimulate release of inositol phosphates from human fibroblasts

The mitogens serum, vasopressin and bradykinin stimulate a significant rise in the inositol phosphate content of cultured human fibroblasts within 10 seconds, while serum- and bradykinin-stimulated arachidonic acid release does not occur until after 30 seconds. The release of inositol phosphates is not secondary to a rise in Ca activity since the Ca ionophore ionomycin does not stimulate release of inositol phosphates. Moreover, we show that phospholipase C in human fibroblasts is activated by these mitogens at resting Ca levels since TMB-8, which blocks the mitogen-induced rise in Ca activity, does not affect the serum-stimulated accumulation of inositol phosphates.

Mitogens and melittin stimulate an increase in intracellular free calcium concentration in human fibroblasts

Intracellular free Ca+2 concentration was measured in cultured human fibroblasts (HSWP) utilizing the Ca+2-sensitive fluorescent probe quin2. The addition of peptide growth factors to serum-deprived HSWP cells induced an immediate rise in intracellular Ca+2 concentration. This mitogen-induced rise in Ca+2 concentration could be blocked by the addition of the intracellular Ca+2 antagonist TMB-8. Addition of the phospholipase activator, melittin, to cells in the absence of growth factors also caused a dramatic rise in intracellular Ca+2 concentration which was blocked by TMB-8.

Ca2+-dependent stimulation of hexose transport by A23187, 12-O-tetradecanoylphorbol-13-acetate and epidermal growth factor in mouse fibroblasts

Ca2+ ionophore A23187 stimulated 2-deoxy-D-glucose (2DG) uptake in Swiss 3T3 mouse fibroblasts. Chelation of extracellular Ca2+ with ethylene-glycol-bis-(beta-aminoethylether) N,N'-tetraacetic acid (EGTA) inhibited the effect of A23187. Similarly, the stimulation of 2DG uptake by a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was prevented by EGTA, whereas the epidermal growth factor (EGF)-stimulated 2DG uptake was not affected by EGTA alone, but in the presence of both EGTA and A23187 which effectively depleted cellular Ca2+ content, EGF could no longer stimulate 2DG uptake. These results suggest that Ca2+ regulates hexose transport system in Swiss 3T3 mouse fibroblasts, the activation of which by TPA and EGF differently depends on Ca2+.

Efflux of 45Ca2+ from human fibroblasts in response to serum or growth factors

Previous studies have indicated that intracellular Ca2+ is involved in fetal bovine serum (FBS)- or growth factor (GF)-stimulated Na+ influx in human foreskin fibroblasts (HSWP). In the present study, 45Ca2+ efflux from serum-deprived HSWP cells was measured in response to 10% FBS or GF [lys-bradykinin, vasopressin, epidermal growth factor, and insulin]. Efflux data were analyzed using a computer program and the best fit indicated the presence of three Ca2+ compartments: a compartment (C1) with a very fast turnover rate, one (C2) with a fast turnover rate, and one (C3) with a slow turnover rate. When serum-deprived cells were treated with 10% FBS, efflux from C2 and C3 increased significantly (p less than 0.05). Similar effects on efflux were observed when serum-deprived cells were treated with individual GFs. Combination of the four GFs produced a higher stimulation than any single factor and a response that was equal to that of FBS. On the other hand, when cells were serum-treated in the presence of the intracellular Ca2+ antagonist, B-(N-N,diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8), 45Ca2+ efflux from C2 was substantially reduced. Finally, when cells were treated with the Na+ transport inhibitor amiloride, there was no significant effect on serum-stimulated Ca2+ efflux. These results are consistent with a FBS- or GF-induced mobilization of Ca2+ that can be blocked by intracellular Ca2+ antagonists, and support the hypothesis that the action of these agents on Na+ influx may be via their effects on intracellular Ca2+.

Lys-bradykinin stimulates Na+ influx and DNA synthesis in cultured human fibroblasts

The effect of Lys-bradykinin on net Na+ influx in serum-deprived cultured human fibroblasts (HSWP cells) was measured. It was found that Lys-bradykinin stimulates net Na+ influx with a K1/2 of 1 nM. When Lys-bradykinin was combined with epidermal growth factor, vasopressin and insulin, the net Na+ influx stimulated was comparable with that measured in response to 10% serum. The above combination of growth factors also was found to stimulate DNA synthesis to 92% of the serum-stimulated levels in HSWP cells and to support cell growth over a period of 6 days. In addition, it was observed that the Na+ influx stimulated by Lys-bradykinin or by the combination of four growth factors was completely inhibited by the amiloride analog benzamil. Thus Lys-bradykinin presumably stimulates the same Na+ transport system as is stimulated by serum. Finally, the present results suggest that an increase in Na+ influx always accompanies DNA synthesis in HSWP cells. On the basis of these observations, it can be hypothesized that Na+ influx is a necessary event to stimulate DNA synthesis.