AT(1)-receptor response to non-saturating Ang-II concentrations is amplified by calcium channel blockers

BACKGROUND: Blockers of angiotensin II type 1 receptor (AT(1)R) and the voltage gated calcium channel 1.2 (Ca(V)1.2) are commonly used for treatment of hypertension. Yet there is little information about the effect of physiological concentrations of angiotensin II (AngII) on AT(1)R signaling and whether there is a reciprocal regulation of AT(1)R signaling by Ca(V)1.2. METHODS: To elucidate these questions, we have studied the Ca(2+) signaling response to physiological and pharmacological AngII doses in HEK293a cells, vascular smooth muscle cells and cardiomyocytes using a Ca(2+) sensitive dye as the principal sensor. Intra-cellular calcium recordings were performed in presence and absence of Ca(V)1.2 blockers. Semi-quantitative imaging methods were used to assess the plasma membrane expression of AT(1)R and G-protein activation. RESULTS: Repeated exposure to pharmacological (100 nM) concentrations of AngII caused, as expected, a down-regulation of the Ca(2+) response. In contrast, repeated exposure to physiological (1 nM) AngII concentration resulted in an enhancement of the Ca(2+) response. The up-regulation of the Ca(2+) response to repeated 1 nM AngII doses and the down-regulation of the Ca(2+) response to repeated 100 nM Angll doses were not accompanied by a parallel change of the AT(1)R plasma membrane expression. The Ca(2+) response to 1 nM of AngII was amplified in the presence of therapeutic concentrations of the Ca(V)1.2 blockers, nifedipine and verapamil, in vascular smooth muscle cells, cardiomyocytes and HEK293a cells. Amplification of the AT(1)R response was also observed following inhibition of the calcium permeable transient receptor potential cation channels, suggesting that the activity of AT(1)R is sensitive to calcium influx. CONCLUSIONS: Our findings have implications for the understanding of hyperactivity of the angiotensin system and for use of Ca(2+) channel blockers as mono-therapy in hypertension.