Proteolysis of myosin during platelet storage.

Physical properties of actomyosin from either fresh or stored platelets have been compared. Actomyosin obtained from platelets after 3 days of storage contained myosin that was 60-80% degraded to myosin rod. No myosin rod was detected in fresh platelets. The platelet myosin rod is similar to the rod produced by limited proteolysis of skeletal muscle myosin.

Know thy neighbor: a survey of diseases and complex syndromes that map to chromosomal regions encoding TRP channels.

On the basis of their ever-expanding roles, not only in sensory signaling but also in a plethora of other, often Ca(2+)-mediated actions in cell and whole body homeostasis, it is suggested that mutations in TRP channel genes not only cause disease states but also contribute in more subtle ways to simple and complex diseases. A survey is therefore presented of diseases and syndromes that map to one or multiple chromosomal loci containing TRP channel genes. A visual map of the chromosomal locations of TRP channel genes in man and mouse is also presented.

Receptor-effector coupling by G proteins.

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Microinjection of the alpha-subunit of the G protein Go2, but not Go1, reduces a voltage-sensitive calcium current.

Calcium currents can be modulated by receptor activation of the GTP-binding protein G(o). We have determined whether the two forms of G(o), Go1 and Go2, differentially regulate calcium current magnitude. Using identified neurons of the pond snail Helisoma, we demonstrate that a high-voltage-activated (HVA) calcium current is reduced by addition of the neuropeptide Phe-Met-Arg-Phe-amide (FMRFamide) and that this inhibition is mediated by a pertussis toxin (PTX)-sensitive G protein pathway. Using this calcium current as an assay for G protein activity, we microinjected GTP gamma S-activated alpha-subunits of G proteins into neuronal somata. We demonstrate that the calcium current is differentially regulated by the two forms of alpha o. Microinjection of alpha o2*, but not alpha o1*, reduces calcium current magnitude.

Temporal characteristics of gonadotropin interaction with rabbit luteal receptors and activation of adenylyl cyclase: comparison to the mode of action of catecholamine receptors.

The temporal characteristics of gonadotropin (LH and hCG) and catecholamine interaction with their luteal receptors and activation of adenylyl cyclase were studied in rabbit luteal membranes. studies on hormone-receptor interaction showed that, once bound, [125I]iodo-hCG dissociated from its receptor very slowly. If excess LH was added 30 min after the initiation of [125I]iodo-hCG binding, 85% of the [125I]iodo-hCG bound at 30 min was still bound to the luteal receptors 4.5 h later. The rate of dissociation of [125I]iodo-hCG from its receptor was not altered by 100 microM GTP, 2 mM MgCl2, or GTP plus MgCl2. The slow rate of [125I]iodo-hCG dissociation observed at 30 min was not due to a time-dependent change in the hormone-receptor complex, as the dissociation of [125I]iodo-hCG was equally slow 5 min after the initiation of the binding reaction. Studies on the activation of luteal adenylyl cyclase by LH showed that stimulation by 1 microgram/ml ovine LH (oLH) could be prevented but, once initiated, could not be reversed by antiserum to oLH. This indicates that once bound to the rabbit luteal LH receptor, oLH causes persistent activation of rabbit luteal adenylyl cyclase. In contrast, the activation of luteal adenylyl cyclase by 1 microM isoproterenol could be completely reversed by the addition of 50 nM propranolol 5 min after the initiation of the reaction. The inhibitory effect of the propranolol could be completely overcome by the addition of excess isoproterenol, indicating that catecholamine binding to its luteal beta-receptor is readily reversible. Thus, there appears to be a basic difference in the mechanism by which the gonadotropins and catecholamines interact with their receptors and activate the rabbit luteal adenylyl cyclase.

Effects of N-ethylmaleimide on gonadotropin and beta-adrenergic receptor function coupled to rabbit luteal adenylyl cyclase.

The effects of the sulfhydryl-reactive alkylating agent N-ethylmaleimide (NEM) on the rabbit luteal adenylyl cyclase system were studied. Treatment of luteal membranes with NEM revealed three activities with differing sensitivities to NEM treatment. When luteal membranes were treated with NEM on ice for 30 min, it was found that NaF-stimulated adenylyl cyclase activity in cholate extracts of these membranes was most sensitive to this treatment. Half-maximal inhibition was obtained at 0.09 mM NEM. The activity of the stimulatory guanine nucleotide- and Mg-binding regulatory component (Ns), as assessed by functional reconstitution of NaF-stimulated adenylyl cyclase activity into membranes from the cyc-variant of the S49 mouse lymphoma, was less sensitive to this treatment, with half-maximal inhibition occurring at 0.69 mM NEM. In contrast, high affinity gonadotropin and beta-adrenergic binding, as assessed by competitive displacement of [125I]iodo-hCG by bovine LH and (-)3-[125I]iodocyanopindolol by isoproterenol, was unaffected by NEM concentrations up to 50 mM when membranes were treated on ice. However, when membranes were treated with NEM at 25 C for 30 min, high affinity gonadotropin and beta-adrenergic binding demonstrated similar sensitivities to NEM treatment, such that 50 mM NEM completely inhibited high affinity binding to both receptors. Under either of the conditions described above, neither the number of receptors nor the affinities of the labeled probes for their receptors were altered by NEM treatment. Thus, there appears to be at least three NEM-sensitive sites necessary for the functioning of the rabbit luteal adenylyl cyclase system, one associated with the catalytic component, one on Ns which interacts with the catalytic component, and one involved in high affinity agonist binding. Furthermore, it appears that formation of the high affinity binding state is regulated similarly for gonadotropin and beta-adrenergic receptors.

Effects of guanine nucleotides and divalent cations on forskolin activation of rabbit luteal adenylyl cyclase: evidence for the existence of an inhibitory guanine nucleotide-binding regulatory component.

The effects of guanine nucleotides and divalent cations on the activation of rabbit luteal adenylyl cyclase by the diterpene forskolin were investigated. Saturating concentrations of forskolin elicited 10- to 15-fold stimulation of adenylyl cyclase activity in the absence of added guanine nucleotide. No lag was observed in the time course of forskolin-induced activation. Addition of 10 microM guanosine triphosphate (GTP) and guanyl-5'-yl imidodiphosphate [GMP-P(NH)P] inhibited forskolin activation by 10-15% and 30-40%, respectively, in the presence of 3.0 mM MgCl2. GMP-P(NH)P was more potent than GTP in inhibiting forskolin activation of adenylyl cyclase having an IC50 of 36 nM compared to 610 nM for GTP. In contrast, the Kact for stimulating adenylyl cyclase activity by both GMP-P(NH)P and GTP were similar, 1.00 and 0.86 microM, respectively. GMP-P(NH)P-induced inhibition of the forskolin-activated enzyme was not due to the hysteretic nature of GMP-P(NH)P activation of luteal adenylyl cyclase, as addition of GMP-P(NH)P to an enzyme that had been treated 5 min earlier with forskolin resulted in the immediate inhibition of enzymatic activity. Addition of GMP-P(NH)P to a concentration-effect curve for forskolin increased the Kact value for forskolin form 7.18 to 26.8 microM. There was a different MgCl2 concentration requirement for maximal stimulation of luteal cyclase by GMP-P(NH)P (8 mM MgCl2) and maximal inhibition of forskolin-stimulated activity by GMP-P(NH)P (0.5-0.6 mM MgCl2). Further, MnCl2 concentrations above 1.0 mM completely abolished the inhibitory action of GMP-P(NH)P on forskolin activation of luteal cyclase. In fact, at 2.0 mM MnCl2, adenylyl cyclase activity in the presence of GMP-P(NH)P plus forskolin was greater than that of forskolin alone. Thus taken together, these findings suggest that the rabbit corpus luteum contains an inhibitory guanine nucleotide-binding regulatory component in addition to a stimulatory regulatory component. Further, these components demonstrate differing requirements for both guanine nucleotides and divalent cations in order to interact with the catalytic moiety of adenylyl cyclase. The existence of such an inhibitory component suggests the presence of an inhibitory receptor in the corpus luteum which could negatively regulate adenylyl cyclase resulting in the inhibition of cAMP production and reduced progesterone output from the corpus luteum under normal physiological conditions.

Human chorionic gonadotropin-induced heterologous desensitization of rabbit luteal adenylyl cyclase is associated with altered receptor and G-protein function.

We injected hCG into pseudopregnant rabbits on day 7 of pseudopregnancy and analyzed changes in the components of luteal adenylyl cyclase system in order to determine which components are responsible for altered hormonal responsiveness upon desensitization. hCG-induced desensitization was homologous (loss of responsiveness to LH) early (first 6 h), then became heterologous (partial loss of responsiveness to catecholamines) later (12-48 h). The total number of LH receptors was reduced approximately 30% 3 h after treatment at a time when LH stimulation of adenylyl cyclase activity was not altered. Total LH receptors remained at this level until 24 h, when total receptors were reduced by 88%. While total LH receptor number remained constant, LH-stimulated adenylyl cyclase activity was declining to 57% of the control value at 12 h. Available unoccupied LH receptors were reduced by 96% at 12 h. The affinity of the occupied receptors was reduced 4-fold before down-regulation. The changes in beta-adrenergic receptor number paralleled the changes in catecholamine responsiveness. hCG treatment also altered luteal G-protein function, as assessed by reconstitution of adenylyl cyclase activity in S49 cyc- lymphoma membranes and ADP ribosylation by cholera and pertussis toxins. Isoproterenol (ISO)-reconstituting activities of luteal Gs (the stimulatory G-protein of adenylyl cyclase) were depressed by 65% 12-48 h after hCG treatment, the same time as reduced catecholamine responsiveness. In contrast, NaF-reconstituting activities were at control levels at 12 h and reduced by 55% at 24 and 48 h. Pertussis toxin's ability to ADP ribosylate alpha i 40 was increased 3 and 6 h after treatment, while cholera toxin's ability to ADP ribosylate alpha s 45 was reduced throughout the study period. These studies demonstrate that hCG-induced heterologous desensitization results in a complex series of changes in beta-adrenergic and LH receptors as well as G-protein function, which account for the altered hormonal responsiveness.

Catecholamine-induced heterologous desensitization of rabbit luteal adenylyl cyclase: loss of luteinizing hormone responsiveness is associated with impaired G-protein function.

The effects of injecting epinephrine into pseudopregnant rabbits on the luteal adenylyl cyclase system were analyzed. Epinephrine-induced desensitization was heterologous and associated with a reduced response to isoproterenol, LH, NaF, and forskolin. Epinephrine-induced desensitization was rapid in onset, with a maximum decrease in responsiveness 6 h after treatment and responsiveness returning to control levels within 24 h of treatment. The changes in beta-adrenergic receptor content paralleled changes in catecholamine responsiveness. The affinity of the beta-receptors from treated animals decreased 1.5- to 2-fold before down-regulation. LH receptor number was not altered by epinephrine treatment, although responsiveness to LH was depressed. LH receptor affinity, however, was reduced about 2-fold by epinephrine treatment. Epinephrine treatment also altered G-protein function in corpora lutea, as assessed by reconstitution of adenylyl cyclase activity in S49 cyc- membranes and ADP ribosylation by cholera and pertussis toxins. NaF- and isoproterenol-reconstituting activities of luteal Gs (the stimulatory G-protein of adenylyl cyclase) were depressed for the first 6 h after treatment. The ability of cholera toxin to ADP ribosylate alpha s 46 and alpha s 45 was reduced 1.5-6 h and 3-12 h, respectively, after epinephrine treatment. The reduced ability of cholera toxin to ADP ribosylate alpha s 45 was associated with the decrease in LH receptor affinity after treatment. This supports the contention that alpha s 45 preferentially interacts with the LH receptor. These studies demonstrate that the loss of LH responsiveness upon epinephrine-induced heterologous desensitization is associated with altered G-protein function.

Ovarian responses of pregnant mare serum gonadotropin- and human chorionic gondotropin-primed rats: desensitizing, luteolytic, and ovulatory effects of a single dose of human chorionic gonadotropin.

We conducted a study to determine the morphological appearance and functional responsiveness of ovarian tissues after administration of hCG to 28-day-old rats primed 65 h earlier with PMS gonadotropin (PMSG) and after administration of a second dose of hCG 5 days later, i.e. to 33-day-old rats containing heavily luteinized ovaries. Sixty-five hours after the administration of 50 IU PMSG sc to 25-day-old rats, ovaries already contained an abundance of luteinized follicles and an adenylyl cyclase (AC) system that was responsive to LH, epinephrine, and NaF. The administration of 50 IU hCG sc at this time initially resulted in a loss of LH-responsive ovarian AC. Within 4 days of the hCG injection, the ovaries of the now 32-day-old rats were heavily luteinized, and ovarian AC was highly responsive to LH, epinephrine, and NaF. The administration of a single sc dose of 200 IU hCG to 33-day-old PMSC- and hCG-primed rats with luteinized ovaries resulted in a rapid desensitization of the ovarian AC to LH and a drop in serum progesterone levels, During the subsequent 7 days, serum progesterone levels continued to decline, while total ovarian AC reacquired responsiveness to LH by days 4--5 after the densensitizing dose of hCG. Dissection of ovarian components revealed, however, that the AC system of the corpora lutea originally present at the time of the second hCG injection remained permanently refractory to LH and that the AC in corpora lutea newly formed from freshly ovulated follicles exhibited a significant responsiveness to LH, epinephrine, and NaF. However, these new corpora lutea were not fully active, since serum progesterone never rose. Subcutaneous administration of 50 IU hCG to 33-day-old PMSG- and hCG-primed rats also promoted a rapid loss of AC responsiveness to LH. This lower concentration of hCG was not sufficient to promote follicular development or ovulation, and the ovarian AC remained refractory to LH for at least 7 days. Intravenous administration of 75 IU hCG to 33-day-old PMSG- and hCG-primed rats similarly promoted a rapid and permanent loss of luteal AC responsiveness to LH; again, follicles did not mature to a preovulatory state and, in fact, appeared to undergo atresia rather than ovulation. These results indicate that in heavily luteinized ovaries 1) hCG promotes desensitization of rat luteal AC to LH, 2) Desensitization of AC to LH stimulation in corpora lutea is permanent and irreversible, and 3) only under conditions where follicles mature and ovulate and new corpora lutea are formed does total ovarian AC reacqure responsiveness during the subsequent week.

The human genome encodes at least three non-allellic G proteins with alpha i-type subunits.

The amino acid sequence and composition of alpha-subunits of signal transducing G proteins of the same kind appear to vary by no more than 2% from species to species. Here we isolated a human liver cDNA using an oligonucleotide complementary to the sequences encoding the pertussis toxin (PTX) ADP-ribosylation site of the alpha-subunit of the rat brain G protein called Gi. Its open reading frame characterizes it as an alpha i-type cDNA--as opposed to alpha o-type--but predicts an amino acid composition that differs by 7% and 14%, respectively, from two other human alpha i-type molecules. Together with human brain alpha i (type-1) and human monocyte alpha i (type-2), the new human liver alpha i cDNA (type-3) forms parts of a family of alpha i molecules. Type-3 alpha i cDNA hybridizes to a approximately 3.6 kilobase long mRNA and type-2 alpha i cDNA hybridizes to an mRNA species of approximately 2.7 kilobases. This indicates that the human genome has at least three non-allellic genes encoding non-alpha o-type PTX substrates and provides structural evidence for the hypothesis that distinct effector systems are regulated by similar but nevertheless distinct PTX substrates.

Guanyl nucleotide regulation of hormonally-responsive adenylyl cyclases.

A large number of hormones and neurotransmitters activate adenylyl cyclase [ATP, pyrophosphate lyase (cyclizing; EC 4.6.1.1.)] catalyzing the formation of cAMP and PPi from ATP in the presence of Mg2+. The cAMP formed is in turn responsible for eliciting the physiological responses of these hormones and neurotransmitters. In addition to hormones and neurotransmitters, fluoride ion, cholera toxin and guanyl nucleotides (GTP and GTP analogs such as GTP gamma S and GMP-P(NH)P) also stimulate adenylyl cyclase activity (Perkins, 1974; Birnbaumer, 1977; Gill, 1977). It has become evident that hormonally-responsive adenylyl cyclase is a multi-component system consisting of at least 3 physically distinct units. The first is the hormone receptor containing a specific site for a given hormone. The second is the catalytic moiety (C component) of adenylyl cyclase bearing the site responsible for catalysis of the cyclizing reaction. The third is the guanyl nucleotide regulatory subunit (G component) which binds guanyl nucleotide. Recently, a GTPase activity has been found to be associated with the G component of adenylyl cyclase (Cassel and Selinger, 1976; Cassel et al., 1977a, b; Lambert et al., 1979). In this review we will present information on the regulation of hormonally-responsive adenylyl cyclases. This is not intended to be a comprehensive review of the literature. Rather, it represents our views on the current status of the regulation of cAMP formation.

Invasive pulmonary aspergillosis complicating influenza A pneumonia in a previously healthy patient.

A rare occurrence of invasive pulmonary aspergillosis complicates influenza pneumonia in a previously healthy adult. Five other similar cases are reported in the literature. Both transient depression of cell-mediated immunity and loss of ciliary function in the tracheobronchial tree occurs during acute influenzal illness and may predispose to fungal superinfection. Early diagnosis and treatment of opportunistic Aspergillus infection complicating influenza is mandatory in view of the high mortality associated with this complication.

Paradoxical effects of thought suppression: a meta-analysis of controlled studies.

Research has shown that attempts to suppress a thought can cause an increase in the frequency of the thought. These paradoxical effects of thought suppression play a key role in cognitive-behavioral models of several emotional disorders. Laboratory studies of this phenomenon, however, have yielded mixed results; and narrative summaries of the literature have not been able to draw firm conclusions about the effects of thought suppression. We used meta-analysis to quantitatively examine the magnitude of thought suppression effects across controlled studies. Moreover, we explored whether the variability in effect sizes could be explained by methodological differences within and between studies. Results indicated a small to moderate rebound effect of thought suppression that varied in magnitude depending on the nature of the target thought and the method by which thought frequency was measured. Participants with clinical diagnoses did not show larger rebound effects than nonclinical or analogue participants, however, only a few studies included clinical samples. Findings are discussed in terms of implications for the ironic process theory of thought suppression, and avenues for future research on this phenomenon.

Angiographic diagnosis and management of head and neck schwannomas.

Schwannomas are tumors derived from nerve sheath cells, which are often located in the head and neck, including the CNS. Although a definitive vascular pattern has been previously characterized for these lesions, preoperative embolization of the more vascular schwannomas has not been described. In a review of eight patients with schwannomas who underwent angiography at our institution since 1987, a characteristic vascular pattern became apparent that helped distinguish these lesions from other lesions of the head and neck. The lesions were moderately vascular with tortuous tumor vessels. Scattered, small puddles of contrast medium seen in the mid-arterial, capillary, and venous phases were believed to be characteristic of these lesions. Multiple feeding vessels were noted in all but one case, but these were only minimally enlarged. No arteriovenous shunting or vascular encasement was identified. Six of eight lesions were embolized with significant devascularization and no morbidity or mortality. In patients with head and neck tumors whose angiographic findings include a pattern of moderate hypervascularity, tortuous tumor vessels, and, in particular, scattered contrast puddles without arteriovenous shunting or vascular encasement, schwannoma should be suspected. Embolization is a useful and safe presurgical adjunct in the treatment of vascular schwannomas.

Effectiveness of psychological and pharmacological treatments for obsessive-compulsive disorder: a quantitative review.

Quantitative review of the controlled treatment outcome literature for obsessive-compulsive disorder (OCD) showed that exposure with response prevention was highly effective in reducing OCD symptoms. Cognitive approaches were also found to be at least as effective as exposure procedures. It appears that both cognitive and exposure interventions involve some overlapping procedures and capitalize on similar mechanisms of change. Serotonergic medication, particularly clomipramine, also substantially reduced OCD symptoms. However, clomipramine may not be particularly superior to other serotonergic medication. The relationship between side effects and effect size in medication trials was explored.

Effectiveness of exposure and ritual prevention for obsessive-compulsive disorder: randomized compared with nonrandomized samples.

The efficacy of exposure and ritual prevention (EX/RP) for reducing symptoms of obsessive-compulsive disorder (OCD) has been demonstrated in several randomized controlled trials (RCTs). However, procedures used in these studies to maximize experimental control may have limited their generalizability to typical clinical practice. Treatment outcome data from 110 clinical patients receiving EX/RP on an outpatient fee-for-service basis were compared with findings from 4 RCTs of EX/RP. Adult patients in the clinical sample were not excluded because of treatment history, concomitant pharmacotherapy, psychiatric comorbidity, age, or OCD severity. Clinical patients achieved substantial and clinically meaningful reductions in their OCD and depressive symptoms following EX/RP, which were comparable with those reported in the RCTs. Findings indicate that EX/RP is a potent treatment for OCD, and its benefits are not limited to select patient samples.

Effect of ACTH and corticosterone on melanogenesis and cyclic nucleotide levels in the B-16 melanoma.

The acute in vitro action of ACTH and corticosterone individually and in combination were determined in the B-16 melanoma grown in vivo. ACTH elevated levels 10-fold while tyrosinase activity generally was depressed. Corticosterone depressed cAMP levels and tyrosinase activity. ACTH in the presence of corticosterone produced a coincident peak in tyrosinase activity and cAMP levels followed by a depression of enzymatic activity. The results demonstrate that cAMP is not the sole modulator of tyrosinase activity and that ACTH, corticosterone and cAMP interact in the regulation of B-16 melanoma tyrosinase activity.

Interaction of ACTH, corticosterone and cyclic nucleotides in Harding-Passey melanoma melanogenesis.

The acute in vitro action of adrenocorticotropin (ACTH) and corticosterone alone and in combination were determined in the Harding-Passey (HP) melanoma grown in vivo. Hormone treated melanoma dice (5--240 min) were analyzed for tyrosinase activity, cyclic AMP (cAMP) and cyclic GMP (cGMP). ACTH elevated cAMP and cGMP levels 20- and 13-fold, respectively, in the HP melanoma. However, these large increases in cyclic nucleotide levels were accompanied by only a 49% increase in tyrosinase activity. Corticosterone elicited a similar response. ACTH plus corticosterone produced an early cAMP and cGMP peak followed by depression. ACTH plus corticosterone stimulated tyrosinase activity coincident with the early cyclic nucleotide peak followed by a drop in tyrosinase activity which was subsequently elevated. The results indicate that neither cAMP nor cGMP are the sole modulators of tyrosinase activity and suggest the interaction of ACTH, corticosterone and cyclic nucleotides in the regulation of melanoma tyrosinase activity.

Acute effects of two melanocytolytic agents, hydroquinone and beta-mercaptoethanolamine, upon tyrosinase activity and cyclic nucleotide levels in murine melanomas.

The acute in vitro actions of two potent melanocytolytic agents, hydroquinone (HQ) and beta-mercaptoethanolamine (MEA), were determined in the B-16, Cloudman S-91 and Harding-Passey (HP) murine melanomas grown in vivo. Drug treated melanoma dice (5--480 min) were analyzed for tyrosinase activity and cyclic nucleotide levels (cAMP, cGMP). HQ and MEA effects on tyrosinase activity are complex and vary with tumor type, duration of treatment and agent tested. MEA or HQ inhibited B-16 tyrosinase activity. With combined drug therapy, low concentrations of MEA plus HQ stimulate B-16 tyrosinase activity while high concentrations of the drugs have little effect on enzymatic activity. MEA depresses tyrosinase activity while HQ elevates enzymatic activity in the S-19 melanoma. Both high and low concentrations of the combined drugs (MEA plus HQ) elicit the same response, stimulation at 10 min followed by continued depression of tyrosinase activity for the remainder of the 4 h study period. MEA initially stimulates HP tyrosinase activity followed by depression of enzymic activity. In contrast, HQ initially depresses HP tyrosinase activity followed by stimulation of enzyme activity. In combination the drugs inhibit HP tyrosinase activity. The effects of MEA and/or HQ on murine melanoma cyclic nucleotide levels are equally complex. MEA or HQ elevate cAMP and cGMP levels in all three tumors with the exception of S-91 cGMP levels which are not altered. In combination the drugs increase cyclic nucleotide levels in each of the three tumor types but at different times. No correlation is present between cyclic nucleotide levels and tyrosinase activity. Thus, the action of increased cyclic nucleotide levels in melanogenesis can not be separated from the direct actions of MEA and HQ upon melanogenesis. The divergent effects of MEA and/or HQ on tyrosinase activity and cyclic nucleotide levels in these melanomas are not correlated with the known in vivo melanocytolytic activity of these drugs. Thus, these parameters appear to be inadequate indicators of melanoma cell viability in chemotherapeutic screening of drugs effective in destroying malignant melanoma.