**5.1. L-Type Ca2+ channel**

Recognized as a universal second messenger in various cellular processes and cell types, Ca2+ signal plays a critical role in many cellular processes, including, but not limited to, gene transcription and excitation-contraction (EC) coupling [91, 92]. Although almost all biological responses are mediated by Ca2+-dependent and Ca2+-controlled processes, Ca2+ signals need to be finely coordinated and precisely regulated. Ubiquitously expressed in the whole body, Cav 1.2 is the main route of Ca2+ entry in VSMCs, essential for vascular EC coupling and control of myogenic tone [93]. As a heteromultimeric channel, L-type Ca2+ channel (LTTC) is formed by four associated subunits: Ca<sup>v</sup> 1.2α1C, Ca<sup>v</sup> 1.2β, Ca<sup>v</sup> 1.2α2δ, and Ca<sup>v</sup> 1.2γ. Undebatable the main subunit, the pore-forming Cav 1.2α1C region, has also been the target of drugs with antihypertensive properties [94–96], although its effectiveness has been achieved only in a subset of hypertensive patients [97]. Importantly Ca<sup>v</sup> 1.2α1C channels are expressed as two distinct tissue-specific transcripts of *Cacna1c* driven by two alternative promoters P1 and P2, encoding, respectively, for a long "cardiac" (Ca<sup>v</sup> 1.2-LNT) and for a short "vascular/brain" (Ca<sup>v</sup> 1.2-SNT) N-terminal region [98]. In VSMCs, LTTC is activated in response to the membrane depolarization, allowing a small fraction of Ca2+ influx, which is sufficient to trigger VSMC contraction. Thus, sustained voltage-dependent Ca2+ influx through the LTCCs maintains a tonic level of vasoconstriction and provides an excitatory template upon which endogenous vasoactive substances may act to modulate arterial diameter and BP.

Although previous studies have demonstrated that aldosterone modulates VSMC Ca2+ currents [99–101], the mechanisms remain to be determined. A landmark study showed that VSMC-specific MR knockout mice (VSMC-MR-KO) are protected against the age-associated rise of BP [42]. Importantly, aged VSMC-MR-KO mice showed decreased myogenic tone and attenuated vascular contraction in mesenteric arteries in response to a LTCC opener. Moreover, mRNA level of Ca<sup>v</sup> 1.2α1C subunit was dramatically downregulated in aortas from aged VSMC-MR-KO mice, suggesting that MR may regulate VSMC Ca<sup>v</sup> 1.2 expression. However, this phenomenon seems indeed to be an age-dependent effect, since a latter study did not validate, at protein level, the downregulation of Ca<sup>v</sup> 1.2 in aortas from young VSMC-MR-KO mice [102]. Furthermore, during the aging process, MR expression increases in resistance vessels along with a decline in the microRNA (miR)-155 abundance, suggesting that Cav 1.2 is a downstream target of miR-155 regulation [103].

Adding more pieces to the puzzle, we recently showed in cardiomyocytes that aldosterone regulates Cav 1.2-LNT by recruiting MR onto targeted genomic regions in "cardiac" *Cacna1c* P1-promoter [19]. Importantly, we deciphered that aldosterone, through MR-dependent mechanism, dramatically activates the "cardiac"-specific *Cacna1c* P1-promoter, even in blood vessels, conferring a new molecular signature to Cav 1.2α1C in this tissue that minimizes Ca2+ channel blocker actions, a mechanism that might participate to treatment-resistant hypertension, as recently proposed [86]. These findings were further validated using a hypertensive rat aldosterone-salt model, as previously described [104]. Although our data showed that aldosterone/MR impairs 1,4-dihydropyridine sensitivity in VSMC through alternative splicing of Cav 1.2α1C, further studies are needed to validate whether this mechanism participates in the resistant hypertension.
