Vascular smooth muscle calcium waves in isolated arterioles : interactions between intraluminal pressure and intracellular calcium handling
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The arteriolar myogenic response, described as the ability of arterioles to regulate their diameter to changes in intra-luminal pressure, is prominent in the control of vascular resistance, autoregulation of blood flow and control of capillary hydrostatic pressure. Despite the myogenic response being first reported by Sir William Bayliss in 1902, the exact signaling mechanisms underlying this mechano-transduction process remain uncertain. Ca2+ is clearly an important second messenger involved in initiating and regulating myogenic contraction. The signaling mechanisms responsible for Ca2+ homeostasis, occurring from both the plasmalemmal ion channels and the intracellular stores, and their subsequent effect on arteriolar myogenic response are poorly understood. Changes in intraluminal pressure in pressurized arterioles apply mechanical force (including stretch) to the smooth muscle cell membrane, which activates unknown mechano- transducers that cause membrane depolarization. Subsequent to this the L-type voltage-gated calcium channels (VGCC) open, leading to Ca2+ entry and activation of a series of signaling pathways that ultimately cause contraction. Apart from the global static elevation in intracellular Ca2+, several other transient Ca2+ events such as sparks, waves, spikes, flashes, sparklets exist in most smooth muscle cells. The relevance of these transient events that may themselves be activated and regulated by intraluminal pressure or by agonist stimulation is still poorly understood. The overall aim of the studies described in this dissertation was to investigate the relationships amongst intraluminal pressure, arteriolar myogenic tone, and transient intracellular Ca2+ regulation with particular emphasis on the mechanisms underlying the generation and propagation of Ca2+ waves.First order arterioles (passive diameter 100 – 200 μm) were dissected from the cremaster muscle tissue of male Sprague Dawley rats. Arterioles were cannulated, pressurized and superperfused with a modified Krebs buffer solution at 34oC and allowed to develop spontaneous myogenic tone. Arterioles were incubated with the Ca2+ sensitive fluorescence indicator, Fluo 4-AM (10 μM) (Molecular Probes, Invitrogen) at room temperature for 1 hour. Arterioles were washed for 30 min with Krebs buffer and allowed to equilibrate and de-esterify the intracellular dye. Some vessels were subjected to removal of the endothelium by passing 0.3% CHAPS through the lumen for 3 minutes followed by a stream of air bubbles to prevent the influence of endothelium dependent factors influencing the smooth muscle. The experiments were conducted in the presence of various pharmacological agents that influence the arteriolar myogenic tone and Ca2+ influx into the cytosol from the plasmalemmal and sarcoplasmic reticulum sources. In initial studies, the effects of intraluminal pressure on Ca2+ waves were examined in the presence and absence of myogenic tone. Increases in intraluminal pressure significantly increased the propagating and regenerative aspects of Ca2+ wave activity, characterized by the number of cells exhibiting waves and the wave frequency, particularly at low pressures. Pharmacological inhibition of myogenic tone by nifedipine (1 μM), adenosine (10 μM) and the MLCK inhibitor ML-7 (10 μM) significantly increased the number of cells exhibiting Ca2+ waves, suggesting that Ca2+ waves are stretch or tension-dependent and exist independently of myogenic tone. Temporal patterns of the waves, assessed by amplitude, rise time, decay time and wave duration were significantly different in the presence and absence of myogenic tone. As Ca2+ influx from the nifedipine sensitive L-type VGCC did not inhibit the Ca2+ wave activity, further studies were conducted to investigate the role of plasmalemmal Ca2+ influx on waves. Pharmacological blockade of the non-selective cation channel family (NSCC) using gadolinium (Gd3+) (10 μM) or SKF-96365 (100 μM) completely abolished the Ca2+ wave activity. This suggests that the Ca2+ influx via the stretch activated channels (SACs)/NSCCs is required for the Ca2+ wave activity. Further, Ca2+ waves were inhibited in the presence of Ca2+-free Krebs containing 2 mM EGTA, and occurred at maximal levels at near physiological extracellular Ca2+ concentrations. Changes in membrane stretch may also be sensed by activation of plasma membrane G-protein coupled receptors (GPCRs) such as α-adrenoceptors. As GPCRs activate phospholipase C (PLC), we observed that U-73122 (5 μM), a potent inhibitor of PLC, abolished the Ca2+ wave activity. Collectively, these data suggest that Ca2+ influx from the SAC/NSCC activated by direct mechanical stretch or by PLC contributes to intracellular Ca2+ wave activity. As Ca2+ waves have been reported to result from cyclical Ca2+ release from the ER/SR, we investigated the exact molecular players involved in the initiation and propagation of Ca2+ waves. Inhibition of the SR Ca2+ ATPase by CPA (30 μM) inhibited the Ca2+ wave activity and increased arteriolar tone. As IP3 produced by the PLC signaling pathway is considered to activate the IP3R, we blocked the IP3R using 2-APB (30 μM) or xestospongin C (10 μM) and inhibited the Ca2+ wave activity. Further, exogenous addition of Bt-IP3 (10 μM) significantly enhanced the Ca2+ wave activity and arteriolar tone. Activation of RyRs using ryanodine (10 μM) increased the global [Ca2+]i and inhibited the Ca2+ waves. However, desensitizing the RyRs by lowering the cytoplasmic Ca2+ content and subsequent activation using ryanodine did not inhibit Ca2+ waves. These data suggest that Ca2+ waves are induced by IP3R activated by the cytosolic IP3 and do not have an absolute requirement for RyR for wave propagation. In addition to studying Ca2+ waves at a mechanistic level, the physiological significance of the Ca2+ waves in the context of arteriolar myogenic reactivity was investigated by assessing temporal aspects of myogenic contraction. Inhibition of Ca2+ waves by ryanodine or xestospongin C decreased the rate of myogenic constriction, while increasing the Ca2+ waves by Bt-IP3 increased the rate of constriction. These data suggest that the presence of Ca2+ waves in arteriolar SMCs provides a basal level of vascular activation, which can be modulated to alter the temporal course of myogenic response. In conclusion, the results of these studies support the pressure-dependency of Ca2+ waves in cremaster muscle arterioles that occur due to Ca2+ entry from the SAC/NSCCs. Ca2+ waves are induced by the IP3R alone and do not require RyR for their propagation. The presence of Ca2+ waves enhances the myogenic reactivity of arterioles. Data presented here improves our current understanding of the mechanisms underlying Ca2+ waves and their possible physiological role. Furthermore, the data also enhances our knowledge of the temporal characteristics of the myogenic response in skeletal muscle arterioles.