02). Likewise, the final recovery of the dim flash response in GCAPs?/?RGS9-ox rods was also ?250?ms ( Fig.?2, A and B), which is much greater than the 80-ms value for ��D in these rods (p?< 0.001). Thus, the acceleration of G?-E? deactivation by RGS9 overexpression had no effect on the final phase of dim flash recovery when GCAPs-mediated feedback was abolished. In sum, in GCAPs?/? rods, the recovery from bright flashes is rate-limited by G?-E? deactivation, whereas the recovery of dim flash responses is governed by a slower process, whose nature we now examine. A fundamental difference between dim and bright flash <a href="http://www.selleckchem.com/products/AZD2281(Olaparib).html">Ku-0059436
datasheet</a> responses is the resulting change in cGMP. Single-photon responses (SPRs) in particular generate relatively small changes in cGMP that vary in both space and time, whereas saturating responses reduce free cGMP effectively to zero homogeneously throughout the entire outer segment. Thus, spatiotemporal dynamics of cGMP cannot affect ��D, but could affect <a href="http://www.selleckchem.com/products/PLX-4032.html">http://www.selleckchem.com/</a>
dim flash response recovery. To investigate how these dynamics might affect SPR recovery, we examined the reactions governing cGMP (cG) by simplifying Eq. 1 to incorporate a constant rate of synthesis (��dark= cGdark ��dark), equation(9) ?cG(x,t)?t=��dark?��darkcG(x,t)+DcG?2cG(x,t)?x2,where ��dark is the rate of synthesis in the dark, ��dark is the rate constant of cGMP hydrolysis in the dark, and DcG is the longitudinal diffusion coefficient for cGMP. During an SPR, the number of G?-E? is expected to have declined to?<10% of their maximum by 600?ms after the flash ( (7a)?and?(7b), with ��R?= 40?ms, ��E?= 200?ms), but in rods with?a constant rate of synthesis (GCAPs?/?), the SPR persists for several hundred milliseconds more ( Fig.?2). At such late times, Eq. 9 has the solution equation(10) cG(x,t)=cGdark?f(x,t)e?��darkt,cG(x,t)=cGdark?f(x,t)e?��darkt,where f(x,t) is a general solution to the one-dimensional diffusion equation whose spatial integral <a href="http://www.selleck.cn/products/Pomalidomide(CC-4047).html">Pomalidomide</a>
is constant in time. If the maximal local change in cGMP, ��cG(x,t)?= cGdark ? cG(x,t), is small, it will be related to the normalized response (r/rmax) via Eqs. 4 and 8 as equation(11) r(t)rmax��3��OS��cG(x,t)cGdarkdx,��e?��darkt.Thus, Eq. 11 predicts that during the tail phase of the SPR, cGMP recovers to its dark level at all spatial locations along a common exponential trajectory with time constant 1/��dark. This explains why the final time constant of SPR recovery in GCAPs?/? rods does not reflect ��D and is unaffected by RGS9-overexpression ( Fig.?2). From this analysis of the tail phase recovery, the rate constant of spontaneous hydrolysis ��dark is thus found to be 1/245?ms?= 4.1 s?1. To our knowledge, this is the first experimental determination of ��dark for mouse rods. The measurement of ��dark made it possible to determine the second factor that determines the spatial spread of the decline in cGMP during the SPR, the longitudinal diffusion coefficient of cGMP (DcG).