Different types of voltage-activated Ca(2+) channels have been established based on their molecular structure and pharmacological and biophysical properties. One of them, the P/Q-type, is the main channel involved in nerve-evoked neurotransmitter release at neuromuscular junctions and the immunological target in Eaton-Lambert Syndrome. At adult neuromuscular junctions, L- and N-type Ca(2+) channels become involved in transmitter release only under certain experimental or pathological conditions. In contrast, at neonatal rat neuromuscular junctions, nerve-evoked synaptic transmission depends jointly on both N- and P/Q-type channels. Synaptic transmission at neuromuscular junctions of the ataxic P/Q-type Ca(2+) channel knockout mice is also dependent on two different types of channels, N- and R-type. At both neonatal and P/Q knockout junctions, the K(+)-evoked increase in miniature endplate potential frequency was not affected by N-type channel blockers, but strongly reduced by both P/Q- and R-type channel blockers. These differences could be accounted for by a differential location of the channels at the release site, being either P/Q- or R-type Ca(2+) channels located closer to the release site than N-type Ca(2+) channels. Thus, Ca(2+) channels may be recruited to mediate neurotransmitter release where P/Q-type channels seem to be the most suited type of Ca(2+) channel to mediate exocytosis at neuromuscular junctions.

Cabilla JP, Ronchetti SA, Nudler SI, Miler EA, Quinteros FA, Duvilanski BH. Nitric oxide sensitive-guanylyl cyclase subunit expression changes during estrous cycle in anterior pituitary glands. Am J Physiol Endocrinol Metab 296: E731-E737, 2009. First published January 13, 2009 doi:10.1152/ajpendo.90795.2008.-17␤-Estradiol (E 2 ) exerts inhibitory actions on the nitric oxide pathway in rat adult pituitary glands. Previously, we reported that in vivo E 2 acute treatment had opposite effects on soluble guanylyl cyclase (sGC) subunits, increasing ␣ 1-and decreasing ␤1-subunit protein and mRNA expression and decreasing sGC activity in immature rats. Here we studied the E 2 effect on sGC protein and mRNA expression in anterior pituitary gland from adult female rats to address whether the maturation of the hypothalamus-pituitary axis influences its effects and to corroborate whether these effects occur in physiological conditions such as during estrous cycle. E 2 administration causes the same effect on sGC as seen in immature rats, and these effects are estrogen receptor dependent. These results suggest that E 2 is the main effector of these changes. Since the sGC ␣-subunit increases while the sGC activity decreases, we studied if other less active isoforms of the sGC ␣-subunit are expressed. Here we show for the first time that sGC␣ 2 and sGC␣2 inhibitory (␣2i) isoforms are expressed in this gland, but only sGC␣ 2i mRNA increased after E2 acute treatment. Finally, to test whether E 2 effects take place under a physiological condition, sGC subunit expression was monitored over estrous cycle. sGC␣ 1, -␤1, and -␣ 2i fluctuate along estrous cycle, and these changes are directly related with E2 level fluctuations rather than to NO level variations. These findings show that E2 physiologically regulates sGC expression and highlight a novel mechanism by which E2 downregulates sGC activity in rat anterior pituitary gland. estrogen; soluble guanylyl cyclase; inhibitory subunit THE MAIN ESTROGENIC HORMONE 17␤-estradiol (E 2 ) plays important regulatory roles in a broad variety of biological processes, acting mainly on reproductive tissues, bone, liver, pituitary, and brain (9, 25).Nitric oxide (NO) is a signaling molecule that freely diffuses across cellular membranes where it binds to its main intracellular receptor, soluble guanylyl cyclase (sGC). This enzyme catalyzes the formation of cGMP from GTP. Subsequently, targets of cGMP such as cGMP-dependent protein kinases, cyclic nucleotide phosphodiesterases, and cyclic nucleotidesensitive ion channels are activated to continue the signal transduction (15,18).sGC is an heterodimeric enzyme and is comprised of two subunits, ␣ and ␤, of which four types exist (␣ 1 , ␣ 2, ␤ 1 , and ␤ 2 ). Both ␣-isoforms form a functional enzyme with the ␤ 1 -subunit, although the ␣ 1 ␤ 1 is the most abundant and widely expressed heterodimer, showing the greater activity (12, 13). The ␣ 2 is expressed in a more restricted pattern: in human tissues, it is present mainly in spleen, placenta, brain, ...

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