Effects of Acute Hypoxia

In pulmonary resistance arteries, it is well recognized that acute hypoxia per se, increases[Ca2+]i in PASMCs in both freshly isolated and cultured cells as well as in intact arterial wall (13, 27, 39, 42). This [Ca2+]i increase triggers the contractile response of PASMCs and thus underlies the subsequent HPV. Perfusion of PASMCs with hypoxic medium quickly increases to its maximum within l-2min, and is maintained during the duration of perfusion (49). Return to anormoxic environment quickly restores the initial [Ca2+]| value. The [Ca2+]j increase is graded from 0 to 500 nM as a function of the severity of hypoxia from Po2 60-80 to 10-15 mmHg (34). A variety of studies have investigated the origin and the mechanism of hypoxia-induced [Ca2+]; increase. Both intracellular and extracellular Ca2+ compartments are involved. It is generally admitted that hypoxia first induces a release of intracellular Ca2+ followed by an influx of extracellular Ca2+ (16, 37). Both voltage-dependent (23, 48) and voltage-independent (e.g., capacitative Ca2+ entry) mechanisms are involved in the Ca2+ influx (27, 34, 40). In addition the SR, mitochondria also seem to play a modulatory role in the hypoxia-induced Ca2+ release from intracellular stores (49). Detailed mechanisms involved in hypoxia-induced Ca2+ release are described in the next chapter (Chapter 6).

In proximal (conduit) arteries, acute hypoxia induces a decrease in the resting [Ca2+]; value, and vasodilation. In one study (48), it has been shown that PASMCs can generate spontaneous Ca2+oscillations that are mainly due to the activation of L-type voltage-dependent Ca2+ channels. Acute hypoxia attenuates and increases the frequency of Ca2+-oscillations in conduit and resistance arteries, respectively. These results suggest that hypoxia modulates the activity of Ca2+ channels in opposite manner in different regions of the pulmonary arterial tree.

Finally, very few information is available about the interactions between acute hypoxia and agonist-induced Ca2+ response. In distal pulmonary artery myocytes from fetal lamb, angiotensin II induces small Ca2+ oscillations (30 nM in amplitude), which are attenuated by acute hypoxia (13).

3.2. Effect of Chronic Hypoxia on Agonist-induced [Ca2+]j Responses

Some aspects ofthe modulation of cellular signaling by chronic hypoxia have been studied on cultured cells maintained in an hypoxic environment for 24 to

72 hrs. However, most ofthe studies investigating the effect of chronic hypoxia on [Ca2+]j responses have been performed on pulmonary arteries obtained from the chronically hypoxic rats, where animals are exposed to a hypoxic environment either under normobaric (10% 02) or hypobaric (0.5 atmosphere) conditions for 2-3 weeks. This procedure induces a selective PAHT with an increase ofthe mean pulmonary artery pressure from ~ 10 to 30 mmHg (Fig. 2A) (10).

Figure 2. Effect of chronic hypoxia (CH) on pulmonary arterial pressure (PAP) and resting [Ca2*], in PASMCs in rats. CH (3 weeks) induced a significant increase in mean PAP (A) and in [Ca2+]j (B) compared with control rats. After 3 weeks of normoxia recovery, mean PAP and PASMC [Ca2+]j were not different from those of control rats. Results are mean±SE. * P<0.05 vs. control.

Figure 2. Effect of chronic hypoxia (CH) on pulmonary arterial pressure (PAP) and resting [Ca2*], in PASMCs in rats. CH (3 weeks) induced a significant increase in mean PAP (A) and in [Ca2+]j (B) compared with control rats. After 3 weeks of normoxia recovery, mean PAP and PASMC [Ca2+]j were not different from those of control rats. Results are mean±SE. * P<0.05 vs. control.

In general, chronic hypoxia increases the resting [Ca2+]j value by -60%, i.e., from 70-75 nM to 115-120 nM (Fig. 2B) (10, 11). This increase disappears in the absence of extracellular Ca2+ but is resistant to organic inhibitors of L-type ,2+

voltage-dependent Ca channels (e.g., nifedipine, verapamil) (11, 45),

suggesting that a different Ca entry pathway may be upregulated by chronic

hypoxia. The exact nature of this Ca influx has not yet been identified.

Figure 3. Effect of chronic hypoxia on agonist-induced [Ca2+]j response in PASMCs. Short (30 s) application of 1 nM phenylephrine (PHE, A) or 0.1 ^M endothelin (ET-1, B), induced oscillations in [Ca2+]i in PASMCs obtained from control (normoxic) rats. The agonist-induced [Ca2+]j oscillations disappeared in PASMCs from rats exposed to hypobaric hypoxia for 3 weeks. In contrast, the amplitude of the caffeine-induced transient [Ca24^ response was not altered by chronic hypoxia (C).

Figure 3. Effect of chronic hypoxia on agonist-induced [Ca2+]j response in PASMCs. Short (30 s) application of 1 nM phenylephrine (PHE, A) or 0.1 ^M endothelin (ET-1, B), induced oscillations in [Ca2+]i in PASMCs obtained from control (normoxic) rats. The agonist-induced [Ca2+]j oscillations disappeared in PASMCs from rats exposed to hypobaric hypoxia for 3 weeks. In contrast, the amplitude of the caffeine-induced transient [Ca24^ response was not altered by chronic hypoxia (C).

As for PAHT in humans, it has been demonstrated that chronic hypoxia in rat increases: i) ET-1 gene expression and plasma ET-1 levels (2, 14, 15, 30) and ii) angiotensin converting enzyme (ACE) expression and activity (32, 33, 51), suggesting that both agonists are implicated in PAHT. Thus, it appears relevant to investigate the effects of chronic hypoxia on Ca2+ responses induced by these agonists. In transiently cultured PASMCs from intrapulmonary arteries, chronic hypoxia largely inhibits the ET-1-induced transient increase in [Ca2+], (45). In freshly isolated PASMCs from extrapulmonary arteries of chronically hypoxic rats, agonist-induced [Ca2+]| responses are drastically altered. The main change is a decrease in the percentage of responding cells (15-30%) and the disappearance ofthe oscillatory profile (Figs. 3 A and B and Fig. 4). This change is not due to a chronic hypoxia-induced change in the Ca2+ source implicated in the [Ca2+]i responses (8). As under control conditions, angiotensin II- or ET-1-induced [Ca2+]| responses in PASMCs from chronically hypoxic rats are not altered in either Ca2+-free solution or by the voltage-dependent Ca2+ channel blocker D600, but vanishes after pretreatment of the cells with thapsigargin.

These results indicate that agonist-mediated [Ca2+]j responses in PASMCs also involve the mobilization of Ca2+ from an intracellular Ca2+ source, presumably the SR (8). In contrast, chronic hypoxia does not modify the percentage of cells responding to caffeine or the amplitude of the Ca2+transient induced by caffeine (Fig. 3C) (8,45). Collectively, these data demonstrate that chronic hypoxia alters Ca2+ oscillations via an action on the IP3-signaling pathway, but not on the CICR mechanism.

Figure 4. Reversal of chronic hypoxia-induced alteration in agonist-induced [Cai+J; response in PASMCs. Endothelin-1 (ET-l)-induced [CaJ+]| oscillations in PASMCs from normoxic (control, A) and chronically-hypoxic (3 weeks, B) rats. The ET-1 -induced [Ca2+]j oscillations were partially recovered in PASMCs from CH rats after 3 weeks of recoveiy in normoxic conditions (C).

Figure 4. Reversal of chronic hypoxia-induced alteration in agonist-induced [Cai+J; response in PASMCs. Endothelin-1 (ET-l)-induced [CaJ+]| oscillations in PASMCs from normoxic (control, A) and chronically-hypoxic (3 weeks, B) rats. The ET-1 -induced [Ca2+]j oscillations were partially recovered in PASMCs from CH rats after 3 weeks of recoveiy in normoxic conditions (C).

3.3. Potential Mechanism of Chronic Hypoxia-induced Disappearance of CaJ+ Oscillation

Chronic hypoxia-induced changes in agonist-induced [Ca2+]j oscillations could result from: i) biochemical modulation of the agonist-receptor binding step resulting in a decrease in the number of receptors expressed at the surface membrane and/or a decrease in the agonist binding affinity, ii) functional alteration of IP3R, and Hi) different IP3R subtypes involved in the response.

In smooth muscle, IP3R is biphasically regulated by [Ca2+]j (26) and this regulation accounts for the cyclical opening and closure of the associated Ca2+ channel (21) and, at least in part, for the so called Ca2+ oscillations. It is unlikely that the amplitude of chronic hypoxia-induced increase in resting [Ca2+]j (-60%, corresponding to « 300 (xM) could modify the negative feedback effect of [Ca2+]i on IP3R. Alternatively, the hypoxia-mediated change in agonist-induced [Ca2+]i oscillation could be due to an alteration in the SR Ca2+ re-uptake mechanism which plays an important role in generating Ca2+ oscillation. Such a Ca2+ reuptake mechanism can be more easily examined by analyzing the falling part of the caffeine-induced [Ca^transients. Although chronic hypoxia does not modify the amplitude of the caffeine-induced [Ca2+]j transients, it significantly decreases the rate of resting [Ca2+]j restoration (8). These results suggest that chronic hypoxia affects Ca2+ reuptake into the SR by the SR Ca2+ pump

(SERCA) and/or Ca2+ extrusion byplasmalemmal Ca2+pump (PMCA) and Na+-Ca2+ exchanger. Indeed, the effect of chronic hypoxia on the kinetics of caffeine-induced [Ca2+]i response is mimicked by CPA, suggesting that chronic hypoxia may inhibit SERCA to delay the recovery ofresting [Ca2+]j. Interestingly, it has been shown in human cardiac muscle that SERCA expression changes during cardiac hypertrophy (1). We believe that the chronic hypoxia-induced decrease or disappearance of agonist-mediated Ca2+ oscillations in PASMCs is due to the downregulation of SERCA. Upon receptor activation, Ca2+re-uptake into the SR is slowed after the first Ca2+ increase during hypoxia; therefore, cytosolic [Ca2+] remains elevated and subsequently modifies the function of IP3R (Fig. 5).

In smooth muscle, as in non-muscle cells, three IP3R isoforms (type 1 -3) are encoded by three different genes (7, 36). Recent studies performed on IP3R reconstituted in lipid bilayer have revealed important functional differences between the three isoforms. Interestingly, type 1-IP3R is biphasically regulated by cytosolic Ca2+ (21, 38,47). Preliminary results from our laboratory show that type 1-IP3R is predominantly expressed in PASMCs from normoxic rats. It is thus tempting to speculate that chronic hypoxia could switch IP3R from a biphasically to a non-biphasically regulated subtype. This hypothesis requires further molecular biological investigations.

Was this article helpful?

0 0
Reducing Blood Pressure Naturally

Reducing Blood Pressure Naturally

Do You Suffer From High Blood Pressure? Do You Feel Like This Silent Killer Might Be Stalking You? Have you been diagnosed or pre-hypertension and hypertension? Then JOIN THE CROWD Nearly 1 in 3 adults in the United States suffer from High Blood Pressure and only 1 in 3 adults are actually aware that they have it.

Get My Free Ebook


Post a comment