Effects of intracellular alkalinization on resting and agonist-induced vascular tone

To evaluate the influence of intracellular alkalinization on basal and agonist-induced vascular tone, we studied the effect of NH4Cl on rat aorta. NH4Cl induced a gradually developing contraction in a dose-dependent manner. Although the contractile response to 20 mM NH4Cl was associated with a latent period (LP) of 23.4 ± 2.8 min, intracellular pH (pH(i)) measurements in cultured rat aortic smooth muscle cells showed that NH4Cl-induced intracellular alkalinization was immediate and transient, returning to basal pH(i) levels in about 30-35 min. Agents that elevate Ca2+, such as A23187 and high KCl, significantly reduced the LP associated with 20 mM NH4Cl-induced contraction. NH4Cl-induced contractions were sensitive to extracellular Ca2+ removal and to the addition of forskolin (1 μM); however, NH4Cl by itself did not cause Ca2+-influx as shown by 45Ca-uptake studies. Addition of 20 mM NH4Cl to precontracted tissues resulted in a transient relaxation, which was complete in ~10 min, followed by a contraction above the original level of tone. NH4Cl pretreatment caused time-dependent alterations in both the rapid and slow phases of phenylephrine and angiotensin II contractions. Rapid-phase responses were diminished at shorter NH4Cl incubation times (10 min), whereas slow-phase response was augmented after a longer incubation (20 min). Overall, the vasorelaxant and vasoconstrictor effects induced by NH4Cl suggest a complex relationship between intracellular alkalinization and arterial contractility.