That most systemic blood vessels alter their tone when the pH of the blood changes is well known. When one asks, however, how this change in tone is produced, it is clear that there is much that is still unknown. As it is the vascular smooth muscle cells which produce the change in vessel diameter, the question becomes, how is an alteration of [H+] outside these cells sensed and transduced into an intra-cellular modulator of contraction? Given that it was more than 100 years ago that Gaskell (1880), studying factors affecting vessel diameter, wrote in The Journal of Physiology that ‘…the state of constriction of muscle of …arteries depends upon the alkalinity of the fluid surrounding them’, it is clear that progress in unravelling this mechanism has not been rapid. In the paper by Iwasawa et al. (1997) in this issue, another important piece of evidence in this puzzle has been obtained.
As the most important determinant of force is intracellular [Ca2+] ([Ca2+]i), this suggests that pHo will affect [Ca2+]i. In turn, changes in [Ca2+]i associated with contraction are brought about by (1) Ca2+ entry and (2) Ca2+ release from the sarcoplasmic reticulum (SR). pHo can affect both of these processes. Figure 1 summarizes how these effects may contribute to the vasodilatation produced by extracellular acidosis (effects on Ca2+ efflux may also occur but are not considered here).
Extracellular protons may antagonize agonist–receptor interactions (Fig. 1, point 1), thereby reducing the production of second messengers and receptor-operated channels (ROCs). Acidosis may reduce the influx of Ca2+ through voltage-operated channels (VOCs) (Fig. 1, point 2). The third mechanism is for protons to affect Ca2+ uptake or release by the SR. It is presumed that this is via a change of intracellular [H+]. The transduction of pHo into a pHi; change occurs rapidly and with little attenuation in some vessels. IP3 binding to the SR and the Ca2+ release channels is pH sensitive (Tsukioka et al. 1994) as is Ca2+ entry through VOCs. Iwasawa and colleagues now add two more actions of external H+ on [Ca2+]i, as indicated by points 4 and 5 on Fig. 1.
Agonists, as well as stimulating Ca2+ entry via VOCs and ROCs, also induce Ca2+ release from the SR of vascular smooth muscle. The depletion of SR Ca2+ stores with agonist stimulation is now known to activate a process that causes further Ca2+ influx–the ‘capacitative’ Ca2+ entry pathway. Calcium currents associated with capacitative Ca2+entry (ICEAC) have been recorded in some cell types. In addition, another pathway for receptor-mediated Ca2+ entry has been identified in vascular smooth muscle cells. This is a channel, permeable to divalent cations, producing currents designated as ICAT. These two Ca2+ entry mechanisms are the focus of the work described by Iwasawa et al. (1997). Their question was, does pHo affect receptor-mediated Ca2+ influx in vascular smooth muscle cells? Their data provide a convincing positive answer to this question.
The authors used the embryonic rat aortic cell line A7r5, and conventional patch-clamp techniques and [Ca2+]i measurements. They systematically examined the effects of pHo on the response of the cells to vasopressin or endothelin-1, with voltage-gated Ca2+ entry being inhibited throughout. Extracellular acidification was found to decrease or even abolish receptor-mediated Ca2+ entry via capacitative Ca2+ entry and agonist-activated ICAT. Alkalinization potentiated Ca2+ entry. The effects on Ca2+ entry were shown not to be due to changes in pH; or [Ca2+]i, nor to the effects of pHo on the permeation of cations through the channel pore or reduction of surface potential. The mechanism may involve receptor-coupled G protein as GTPγS was able to activate ICAT in a pHo-sensitive manner, in the absence of agonists.
One piece of data missing from the current study was the detection of ICRAC, despite the stimulation of capacitative Ca2+ entry. Previous workers have also failed to detect this current in vascular smooth muscle. Is this because ICRAC is extremely small in these cells (< 1 pA cell-1) or because they have a different capacitative type of mechanism?
Perhaps the final question which remains is how applicable studies on a cell line may be to vascular smooth muscle in vivo and in general? It is entirely likely that, just as there are differences between vascular beds in how much pHo affects pHi, the quantitative contribution of the mechanisms outlined here may also differ. Indeed, acidosis does not cause vasodilatation in all vascular beds, and in some smooth muscle external alkalinization inhibits Ca2+ entry and force production (Heaton et al. 1992). Therefore further mechanisms must remain to be elucidated. Whatever these are, the present experiments of Iwasawa et al. (1997) are an important step in our appreciation of how extracellular pH affects vascular tone.
Due to space limitations only additional references to those in the accompanying paper are given.