Contractile responses to agonists were investigated using basilar arterial strips without endothelium that were isolated from the rabbit cisterna magna double-injection SAH model. Thrombin levels at even 10 U/ml in the basilar artery in the control model induced only a small contraction, whereas contractions were significantly enhanced at a lower concentration in the SAH model [11, 14]. Enhanced contractile response was also observed with an agonist peptide for the thrombin receptor after SAH [14]. Consistent with the increased contractile response to thrombin, expression of the thrombin receptor, proteinase-activated receptor 1 (PAR₁), was upregulated 5 and 7 days after SAH [14]. Intrathecal administration of a selective PAR₁ antagonist prevented both the upregulation of PAR₁ expression and enhancement of the contractile response to thrombin [7]. This suggests that thrombin-mediated activation of PAR₁ plays a critical role in upregulating the expression of PAR₁ itself, thereby enhancing the contractile response to thrombin after SAH. A similar enhancement of the contractility was also observed with platelet-derived growth factor, phenylephrine, and ET-1, but not for high K⁺-depolarization or phorbol ester [11, 14]. The expression of PAR₁, α1-adrenoceptor, and ET_A receptor has also been found to be up-regulated after SAH [11]. On the basis of these findings, receptor upregulation is suggested to play an important role in the increased vascular reactivity to agonists.

Contractile responses to agonists usually diminish during persistent or repetitive stimulation with agonists. These phenomena are referred to as desensitization or tachyphylaxis, respectively, and represent an important physiological feedback mechanism that protects against both acute and chronic receptor overstimulation [15, 20]. In actuality, the mechanism of receptor desensitization is impaired under various pathological conditions, such as hypoxia, cancer, and diabetes [1, 3, 4]. Thus, the attenuation of contractile responses through desensitization or tachyphylaxis may help to prevent the development of cerebral vasospasm.

In the control model, the agonist stimulation of rabbit basilar artery with ET-1, thrombin, and phenylephrine induced a transient contraction that reached a peak and then gradually declined to the significantly lower level [11]. This attenuation of the contractile response after persistent receptor stimulation is consistent with desensitization. On the other hand, the transient contractile response was converted to a sustained response in the SAH model [11]. A similar conversion of contractility following SAH was also observed for stimulations with angiotensin II or vasopressin [9]. This means that receptor desensitization was impaired after SAH. The conversion of the transient response to the sustained response was also observed with cytosolic Ca²⁺ concentration ([Ca²⁺] _i ) and myosin light chain (MLC) phosphorylation [11]. Furthermore, when the artery was consecutively stimulated with PAR₁-activating peptide, phenylephrine, angiotensin II, or vasopressin, the response to the second stimulus was significantly reduced in the control model. This attenuation of the contractile response to the second stimulation is consistent with tachyphylaxis. On the other hand, the contractile response to the second stimulation was well preserved in the SAH model [9, 11]. This means that tachyphylaxis was impaired after SAH. These observations suggest that, after SAH, the feedback regulation mechanisms of the receptor-mediated contraction, such as desensitization or tachyphylaxis, were impaired upstream of the Ca²⁺ signal, and presumably at the receptor level, thereby causing a sustained contraction and persistent response to the second stimulation.

The impaired feedback regulation may cause a significant influence on the contractile effect of thrombin because of the unique activation mechanism of PAR₁. The activation of PAR₁ by thrombin is initiated by proteolytic cleavage of the N-terminal region, which covers the region that acts as a tethered ligand and activates the receptor [2]. Feedback regulation therefore plays an important role in terminating the activity of the proteolytically activated PAR₁. In the SAH model, thrombin-induced sustained contraction was found to persist even after terminating the thrombin stimulation [11]. Trypsin is known to remove the ligand region of PAR₁, thereby converting the active conformation of PAR₁ to the inactive conformation [16]. The addition of trypsin during the thrombin-induced sustained contraction completely inhibited the contraction [11]. Furthermore, an inhibitor of the Gαq protein also inhibited the thrombin-induced sustained contraction in the SAH model [11]. These observations therefore suggest that the persistent contraction is associated with the persistent activation of PAR₁, and that the feedback inactivation of PAR₁ is impaired following SAH. The Gαq inhibitor also inhibited the sustained phase of the contraction induced by ET-1 and phenylephrine [11]. These findings suggest that impairment of the feedback regulation of the receptor activity is not limited to PAR₁, but also extends to other receptors. This general impairment of the receptor inactivation may explain the enhanced contractility to the various agonists that is observed following SAH.